One of the many exciting challenges facing new and seasoned aquarists alike, is the process of building an aquarium community of creatures.  The Aquarist must have a forward looking perspective to anticipate how a new addition will interact with the aquarium décor, and its’ other inhabitants.   Just as in life, we get along with some more than others.  Our aquatic friends are no different.

Unlike us, aquatic creatures work off of propensity and instinct, weaving their behaviors together in a complicated dance of life and death.  In the aquatic realm the phrase ‘survival of the fittest’ is quite literally a mantra of existence.  One’s instinctual behavior becomes another’s trigger for attack and likely consumption.  And so, life in the aquatic realm is complicated and brimming with uncertainty.

The job of the aquarist is to understand the instinctual actions of potential aquarium occupants, placing creatures together that exhibit similar or complimentary behaviors.

Behavior is a complicated realm and often unable to stand alone as the sole consideration for compatibility.  It seems that animal behavior is greatly affected by the environment in which the animal exists.  Although an aquarium occupant my have a propensity for burrowing in a sandy floor, an aquarium without a sandy floor will inhibit the occupant from exhibiting this behavior.  If burrowing is tied to feeding, the occupant may not engage in their primary feeding method, thus affecting their general health due to malnutrition.

It is this complicated set of interdependencies which aquarium occupants exhibit that is the impetus for this guide.

Determining compatibility between aquarium occupants is difficult.  This is not because of the inability to align similarities between occupants.  Rather, the difficulty stems from the aquarists’ expectation to see many varied aquarium occupants in a relatively small cubic area.

From the aquarist point of view, the aquarium should be teeming with many varied occupants.  Wherever and whenever an aquarist looks into the aquarium, there should be an occupant performing some activity.

After all, the whole setup is rather expensive and there should be some pleasure derived from watching animal behavior.

From the aquarium occupant’s point of view, the more occupants in the tank, the more threats and uncertainties there are that must be responded to.  Overcrowding an aquarium represents a significant threat to existing occupants.

So, there is an inverse relationship between the aquarist’s expectations, and the aquarium occupants’ threat levels.  To solve this dilemma the aquarist must work within the aquarium environment constraints.  If the aquarium has limited volume (30 gals) and is physically small, then the selection of an active fish that can grow to 14″  is probably not appropriate and would create discord and incompatibility with a school of 10 guppies who would soon become meals to the larger fish.

However, an aquarium with 250 gallons and many hiding locations might change the compatibility between the two species.  The guppies would be able to hide from the larger fish and behave more like guppies and less like an afternoon meal to the larger fish.

In the aquarium biome, compatibility is largely comprised of four areas.  They are: Food types, Water chemistry, Species behavior, and Environmental.   The closer there is alignment in these areas and their sub-elements, the greater the compatibility potential.  Many aquatic occupants will overlap in these areas, or not be specific to a particular element in an area.

However, some occupants are highly specific to an element in an area.  It might be a food type, or a behavior that is unique, specialized and species specific.  These occupants may either dictate an environmental condition be met, or food be provided.  Generally, they are compatible with most other aquarium occupants unless the environmental requirement is too far out of bounds for the balance of the aquarium occupants.

To gain a better understanding of compatibility an examination of the four general areas is needed.  A short discussion is helpful to clarify distinctions between elements and identify attributes which an aquarium occupant might have.

For aquatic creatures, there is a wide variety of different food types available.  Generally speaking, if it’s organic and it’s in the water, it’s a food type.  Fish and other aquatic animals will consume just about anything that makes it’s way into the water.

From a compatibility standpoint, the food type used for the aquarium occupants should be palatable to all.  Trying to provide different food types for different species’ palates is a difficult task unless the food types are live and size differentiated.  A size differentiated food type will appeal to larger fish and not to smaller fish.  Consequently it might be possible to target a particular aquarium occupant with a type of food.

For the purposes of establishing compatibility, the aquarist should verify the aquarium occupants are able to consume the same food types.  A helpful tool in this process is the Food Type Quad Chart.  The chart allows the aquarist to plot the different types of food that are consumed by the existing aquarium occupants.  The aquarist then has the ability to see what food types are available for a potential addition to the aquarium.

To use the Food Type chart, the aquarist will identify the food types consumed by existing aquarium occupants by placing a mark for each occupant along the line emanating from the center of the chart and intersecting with the normalcy circle.  This is done for each occupant.  The result will be a series of dots along the circumference of the circle indicating what foods are consumed.

To determine compatibility the potential new occupant is likewise plotted in the chart to see if the food requirements match.

Following is a short discussion on food types commonly used for aquariums.  Some are added to the aquarium, and some develop in the aquarium over time.

Food flakes are generally available in most pet stores and are typically composed of a mixture of ingredients that provide essential nutrients for fish. The specific composition can vary depending on the brand and type of fish food, as different fish species have varying dietary requirements. Generally, they are composed of the following:

• Fish Meal: Fish meal is a primary source of protein in fish food flakes. It is made from ground-up fish and is rich in essential amino acids.
• Wheat Flour or Cornstarch: These ingredients are often used as binders to hold the flakes together.
• Soybean Meal: Soybean meal is another source of plant-based protein used in some fish food formulations.
• Fish Oil: Fish oil is a source of essential fatty acids, particularly omega-3 fatty acids, which are important for fish health.
• Wheat Germ: Wheat germ is a source of vitamins and minerals, including vitamin E.
• Spirulina: This blue-green algae is sometimes added to fish food for its rich nutrient profile, including vitamins, minerals, and pigments.
• Vitamins and Minerals: Various vitamins and minerals are added to ensure a balanced diet for the fish. These may include vitamin A, vitamin C, vitamin D, calcium, and others.
• Color Enhancers: Some fish food flakes contain additives to enhance the color of ornamental fish, such as carotenoids like astaxanthin.
• Preservatives: To extend the shelf life of the flakes, preservatives like ethoxyquin or vitamin C (ascorbic acid) may be included.
• Antioxidants: Antioxidants like BHT (butylated hydroxytoluene) can be added to help maintain the freshness of the product.
• Binders and Stabilizers: These are used to maintain the shape and texture of the flakes and can include ingredients like guar gum or agar.

Generally, fish flakes are available at most pet stores and Internet sellers. There is some variability in contents to more closely address nutrition requirements of different fish species.ntially incompatible species.

Plant Matter & Algae:  Plant matter and algae can be important components of fish diets, particularly for herbivorous and omnivorous fish species. These sources of food provide essential nutrients, fiber, and natural pigments.  Generally, plant matter can be any part of a plant that is either still attached to a living rooted plant, or material that has recently detached from a plant.

• Leafy Greens: Many herbivorous fish, such as certain species of cichlids and plecos, thrive on leafy greens like lettuce, spinach, and kale. These greens provide essential vitamins, minerals, and dietary fiber.
• Vegetable Slices: Cucumber, zucchini, and peas are often used as fish food. These vegetables are typically blanched or softened to make them more digestible for fish.
• Algae Wafers: Algae wafers are specially formulated fish food that contains a high proportion of plant-based ingredients like spirulina, algae, and various vegetables. They are designed for herbivorous and omnivorous fish.
• Seaweed Sheets: Some marine fish, like tangs and surgeonfish, require seaweed or algae sheets as a significant part of their diet. These sheets can be attached to clips inside the aquarium.
• Aquatic Plants: In aquariums with live plants, fish may nibble on the plants. While this is not their primary source of food, it can supplement their diet and provide natural grazing opportunities.

Initially this food source is difficult to provide in a new aquarium.  However after a month or so, there will be ample plant matter and algae to support occupants who favor this food element.

Invertebrates: Invertebrates are a common and highly nutritious source of food for many fish species, both in the wild and in aquariums. Invertebrates encompass a wide range of organisms that lack a vertebral column, including various types of aquatic animals and insects. They are an essential part of the natural diet for many fish species and can be used to supplement the diets of fish in aquariums.

• Brine Shrimp (Artemia): Brine shrimp are tiny aquatic crustaceans that are commonly used as live or frozen fish food. They are rich in protein and are particularly suitable for feeding small fish, fry, and picky eaters. Brine shrimp are often hatched from cysts and used as live baby fish food.
•  Daphnia: Daphnia, often referred to as “water fleas,” are small planktonic crustaceans. They are a nutritious and natural food source for many fish species, including freshwater and marine varieties. Daphnia can be collected from natural bodies of water or cultured for use as live or frozen fish food.
• Bloodworms (Chironomidae): Bloodworms are the larvae of certain types of midges. They are often available as frozen or freeze-dried fish food and are highly attractive to many fish due to their red coloration and high protein content.
• Tubifex Worms: Tubifex worms are small, reddish aquatic worms found in the sediments of freshwater habitats. They are a popular live or frozen food choice for many aquarium fish. However, it’s important to ensure that tubifex worms are from reputable sources to prevent the transmission of diseases to fish.
• Blackworms (Lumbriculus variegatus): Blackworms are freshwater annelids that are used as live or frozen fish food. They are a good source of protein and are often fed to fish that enjoy hunting for live prey.
• Mysis Shrimp: Mysis shrimp are small shrimp-like crustaceans found in both freshwater and marine environments. They are commonly used as frozen or freeze-dried fish food and are particularly popular in marine aquariums.
• Insect Larvae: Some fish, especially carnivorous and insectivorous species, enjoy a diet that includes insect larvae. This can include various types of fly larvae, mosquito larvae, and other aquatic insects.
• Snails: In some cases, certain fish species will consume small snails as part of their diet. This is more common in species like pufferfish and some loaches.
• Copepods: Copepods are small crustaceans found in both marine and freshwater ecosystems. They are a valuable live food source for many marine and some freshwater fish, particularly marine reef species.

This food element develops over time in most aquariums.  However, if your occupants require this food element right away most pet stores can accommodate your requirements.  If needed check online for suppliers of invertebrates.  It is recommended that the aquarist consider non-living invertebrates initially, or at least until the aquarium begins supporting invertebrates on its’ own.

Biofilm, Protozoa, insect larvae: Biofilm, protozoa, and insect larvae are natural food sources that can be valuable components of the diet for many fish species, both in their natural habitats and in aquariums. These organisms offer a diverse range of nutrients and are often sought after by fish for their high protein content and palatability.

1. Biofilm:

• What is Biofilm: Biofilm is a complex, slimy, and naturally occurring community of microorganisms that forms on various surfaces in aquatic environments, such as rocks, substrate, and plants. It consists of bacteria, algae, fungi, and other microorganisms.
• Fish Diet: Many fish species, particularly herbivores and some omnivores, graze on biofilm as part of their natural diet. Biofilm can provide a source of protein, carbohydrates, and other essential nutrients for these fish.
• Use in Aquariums: In aquariums, biofilm can develop naturally on surfaces like rocks and driftwood. Some aquarists intentionally encourage biofilm growth by allowing surfaces to develop it, providing a supplemental food source for their fish.

2. Protozoa:

• What are Protozoa: Protozoa are single-celled microorganisms that are abundant in aquatic environments. They come in various shapes and sizes and are a diverse group of microorganisms.
• Fish Diet: Many fish, including small fry and some filter-feeding species, consume protozoa as a primary or supplemental food source. Protozoa are rich in protein and can be an essential part of the diet for some fish.
• Use in Aquariums: In aquariums, live protozoa cultures can be fed to fish, especially small or fry-stage fish that benefit from tiny food particles. Culturing protozoa at home is possible and can provide a steady supply of live food.

3. Insect Larvae:

• What are Insect Larvae: Insect larvae are the immature stages of insects, such as flies, mosquitoes, and beetles. They come in various forms and sizes, depending on the species.
• Fish Diet: Insect larvae are a highly sought-after natural food source for many fish species. They are rich in protein and essential nutrients. Fish that feed on the water’s surface or near the bottom often consume insect larvae.
• Use in Aquariums: Insect larvae can be offered to aquarium fish as live, frozen, or freeze-dried food. Drosophila (fruit fly) larvae, mosquito larvae, and blackworms are examples of insect larvae used as fish food. Some aquarists also culture their own insect larvae as a sustainable food source for their fish.

Water chemistry is a critical factor in maintaining a healthy and stable environment in aquariums. It involves understanding and managing various chemical parameters of the water.  Since an aquarium is a defined space, it is essential that all occupants in an aquarium be able to tolerate and hopefully thrive in the same aquatic biome.

To better understand salinity and water hardness the Water Chemistry chart allows the aquarist to plot an aquarium occupant’s preferences.  In this area of compatibility all occupants should be closely plotted together.  A potential new addition would need to be in the same quadrant as the other occupants in the aquarium for there ti be any hope of compatibility.

In this plotting chart salinity is presented as a gradient from zero parts per thousand (PPT) at the bottom up through 40 ppt at the top.  Anything from 0 to 1 ppt is considered freshwater.  Some scientists argue for an upper limit of 0.5 ppt as the upper limit of salinity to freshwater.  Saltwater is anything above 30 ppt.

Anything between 30 ppt and 1ppt is considered brackish water.  Within brackish water there are three levels of low, medium, high.  Their ppt designations vary considerably.  Consequently, those segregations are not presented in the compatibility chart.

An aquarium occupant will exist somewhere in the spectrum of 0 ppt to 40 ppt.  Depending on the aquarium, the water chemistry salinity will match with the potential new occupant.

Salinity of water: Another element of water chemistry is the salinity value of the water. Salinity levels, which refer to the concentration of dissolved salts in water, can have a significant impact on aquarium fish, particularly for those kept in marine or brackish water setups.

Different fish species, and other aquarium occupants, have varying tolerance levels for salinity.  Maintaining the appropriate water salinity is essential for their health and well-being.  There are three different classifications used to describe the salinity of aquarium water, and naturally present water in the wild.  They are Freshwater, Brackish water, and Marine water (saltwater).

The exact amount of dissolved salts that each classification uses as a definition varies slightly (.5 ppt), but generally there is a fairly firm consensus on what constitutes freshwater and salt water (marine).

Freshwater vs. Marine vs. Brackish Fish:

• Freshwater Fish: Freshwater fish come from environments with low salinity levels, typically less than 1 part per thousand (ppt) of dissolved salts. They are adapted to this low-salinity environment and can’t tolerate high salinity levels.
• Marine Fish: Marine fish are adapted to high salinity levels, usually ranging from 30 to 40 ppt or even higher, depending on the species. They have specialized salt-regulating mechanisms to maintain their internal salt balance in seawater.
• Brackish Fish: Brackish water fish inhabit estuaries or areas with varying salinity levels, typically ranging from 1 to 30 ppt. They can adapt to a wide range of salinities but often have preferred salinity levels within this range.

The overarching difference between aquarium occupants and their ability to handle dissolved salts is a biochemical process called osmoregulation.  Osmoregulation is a vital physiological process that enables organisms to survive and thrive in a wide range of environments by maintaining the balance of water and solutes in their bodies.  Osmoregulation mechanisms can vary widely among different organisms, depending on their habitat.

Osmoregulation:

• Marine Fish: Marine fish, which live in high-salinity environments, actively pump excess salts out of their bodies through specialized cells in the gills, while conserving water and actively transporting ions like sodium and chloride into their tissues.
• Freshwater Fish: Freshwater fish, which live in low-salinity environments, actively take in salts through their gills and excrete large amounts of dilute urine to prevent water loss.
• Marine Invertebrates: Many marine invertebrates, including crustaceans and mollusks, have specialized structures like gills, nephridia, or specialized cells to manage osmoregulation according to their specific needs.

Compatible aquarium occupants will have the same or similar salinity tolerance.  The impact of placing an occupant incorrectly in a high salinity or freshwater aquarium can be deadly.  Consider the following situations:

• Freshwater Fish in High Salinity: If freshwater fish are exposed to high salinity levels, they will struggle to expel excess salts, leading to dehydration and stress. Prolonged exposure can be fatal.
• Marine Fish in Low Salinity: Marine fish placed in freshwater or brackish water may have difficulty maintaining their internal salt balance, leading to stress, organ dysfunction, and eventually death.
• Brackish Fish in Inappropriate Salinity: Brackish water fish may adapt to different salinity levels, but abrupt changes can stress them. It’s essential to gradually acclimate them to new salinity conditions.

Water hardness, specifically the measurement of dissolved minerals, can significantly impact the health and well-being of aquarium occupants, particularly fish and invertebrates. The two main aspects of water hardness are general hardness (GH) and carbonate hardness (KH).

For the purposes of species compatibility analysis, the GH or degree of GH(dGH) is used in this guide. Carbonate hardness (KH) relates more towards the stabilizing effect Carbonate and bicarbonate ions have against pH variations in the aquarium water.

Some aquatic occupants are highly sensitive towards changes in pH. And that sensitivity is addressed under pH.

For more information regarding water hardness, visit:  SNIPPET: What is Hard Water.

dGH (Degree of General Hardness):

• Calcium and Magnesium Content: GH primarily measures the concentration of calcium and magnesium ions in the water. These minerals are essential for the growth and overall health of fish and invertebrates. A lack of calcium and magnesium can lead to various health issues.
• Impacts on Fish: In soft water (low GH), fish that require higher levels of calcium and magnesium may suffer from deformities, weakened bones, and poor growth. Conversely, in hard water (high GH), fish adapted to soft water conditions may struggle with stress and reduced reproductive success.
• Invertebrates: Many aquatic invertebrates, such as snails and crustaceans, rely on calcium to build and maintain their shells or exoskeletons. In soft water with low GH, these organisms may have difficulty forming and maintaining their protective structures.

dGH Impact on Specific Species:

• Species Variability: Different fish and invertebrate species have varying preferences for water hardness. It’s crucial to research and understand the specific requirements of the species in your aquarium. Some species are adaptable and can tolerate a range of hardness levels, while others are highly specialized.
• Breeding and Reproduction: Water hardness can influence the breeding and reproduction of fish. Some species require specific hardness levels to trigger spawning behavior or to raise healthy fry.

Acclimation: When introducing new fish or invertebrates to an aquarium, it’s essential to acclimate them gradually to the water’s hardness and other parameters. Sudden changes in water hardness can stress or harm aquatic organisms.

The degree of hardness of the aquarium water is represented in the Water Chemistry compatibility chart across the chart from left to right.  The dGH is zero at the far left and rises  to 17 dGH at the far right.  Aquarium occupants will have a preference for water hardness, and that preference should be reflected in the aquarium water chemistry.  For compatibility purposes, all aquarium occupants should be within 4 points of one another.

In addition to salinity and water hardness, pH and water temperature have crucial roles in determining aquarium occupant compatibility.  The Water Chemistry 2 chart depicts two opposing scales pf pH and Water temperature.

The pH scale (described below in detail) is represented along the vertical axis with 7.0 pH centered on the vertical axis.  The entire pH scale is not presented.  Rather, a portion of the scale from 5.0 – 9.0 is presented.  This portion of the pH scale is where most aquatic species exist.

Along the bottom of the chart is water temperature scale.  The scale represented is from 60 F – 90 F.  This temperature range is the most common range for aquariums.  However, if the aquarist is configuring a specialty tank for occupants of colder climates, the range would need to be adjusted to reflect a different temperature range.  The median temperature shown is 75 F.

To use this chart the aquarist would plot their existing aquarium occupants’ pH preferences and their optimum temperature.  The intersection of the two components would represent the existing occupants tolerances.  A new occupant would likewise be plotted.  The representative plots will most likely be lines, as there is usually a range of tolerated pH and temperatures that aquarium animals can tolerate.

To determine compatibility the aquarist would look for intersecting lines between the existing occupants and the proposed new occupants.  If the lines do not intersect or are a distance apart, then the likelihood of achieving good compatibility between occupants is low.

The pH scale ranges from 0 to 14, with 7 being neutral. A pH value below 7 indicates acidic conditions, while a pH value above 7 indicates alkaline or basic conditions. The scale is logarithmic, meaning that each whole pH unit represents a tenfold difference in acidity or alkalinity. For example, water with a pH of 6 is ten times more acidic than water with a pH of 7.  Most occupants and fauna in the wild exist within a point, or at most  two points, of 7.0 pH.

It is important to select potential aquarium occupants that share the same pH tolerance.  Many aquatic creatures can survive in slight variations from their preferred pH environment, but will not thrive.  If the aquarist wants to breed their occupants and otherwise enjoy long healthy lifespans of their aquarium occupants, it is critical that pH be closely monitored and adjusted appropriately to maintain the desired pH level.

pH also contributes to overall aquarium health and acts as a limiting agent against algal blooms, bacterial blooms, and many water-borne pathogens.

The Water Chemistry quad graph depicts pH along the left axis and has a range of 5.0 pH to 9.0 pH.  This allows for 2 points on either side of neutral 7.0 pH.  To be compatible an aquarium occupant should match other occupants as close as possible.  If the new occupant is outside the dotted ring (represents a full point on the pH scale either side of 7.0 pH), the aquarist should reconsider their occupant selection.

Similar to pH, temperature of the water is equally important to compatibility among aquarium occupants.  The typical range for aquarium fish is 70 F – 80 F.  However, there are some species that prefer much cooler temperatures in the 50 F – 60 F range.

Combining aquarium occupants with large ambient temperature variances is not recommended.  For compatibility, occupants should be within 5 degrees of their preferred temperature. It is possible for occupants to survive larger variances up to 10 or more degrees.

Keep in mind that large temperature differences between occupants will have a stressful effect.  With increased stress comes a general inclination towards poor health and the inability of the aquarium occupant to fight off pathogens.

All aquarium occupants will to some degree or another have behavioral attributes.  These attributes are generally instinctual in nature and not learned.  However, some behaviors can be learned and often build upon an occupant’s instinctual behaviors.  So, a fish that is instinctually predisposed to reclusivity may at times exhibit schooling behavior when a predator is present.  There is safety in numbers.

Behavior can also change or be muted due to environmental constraints.  A Loach’s instinctual behavior is to rummage through the bottom sand for food.  If the aquarium has no sand the behavior of the Loach will change to just prowling along the aquarium floor for food scraps, not indulging in any burrowing behavior.

Age is also a behavioral determinant.  The frye of fish, or young fish in general, do not exhibit the behaviors of mature adults.  As a fish matures instinctual behaviors that are species specific will become increasingly evident.

With the many variations of behavior among fish, how does one map the behavior of one species against another to determine compatibility.  A useful tool in this process is the behavior quadrant.

The behavior quadrant has four behavior elements positioned across from their opposite attribute.  There is an implied scale from the center of the diagram to the outer side for each attribute.  The center representing no presented behavior for an attribute,and the outer edge indicating a strong presentation of the behavioral attribute.

There is a lighter circle that runs through each attribute area representing what would be a normal presentation of a behavior attribute for an aquarium occupant.  This is the normalcy curve and represents a normal behaving occupant.

The behavioral area has two groups of elements which oppose one another.  The two groups are Aggressive:Calm, and Social (schooling):Reclusive(hiding).  All occupants have these attributes.

The behavioral elements are positioned in accordance with opposition plotting.  To use the chart the aquarist would plot an existing occupant’s aggressivity using the vertical axis.  The occupant is either calm or aggressive.  The plot would be located within the circle unless the occupant is unusually aggressive or unusually clam.  In these instances the plot made by the aquarist would be outside the normalcy circle.

The aquarist would then plot the social nature of the occupant as either very social or very reclusive.  Using the normalcy circle as a guide the plot would be on or within the circle.

To determine compatibility, the aquarist would then plot a potential new occupant to see if they exist on an opposing side of the quadrant element on the circle for the opposing element.   The opposing side would indicate the new occupant will not interfere with an existing occupant.

The general rule for using the behavioral quadrant is to look for species that will take a place in the quadrant that is opposite to the other species in the aquarium.

As an example Neon Tetras are schooling fish that are social and enjoy swimming among plants.  These fish would be well paired with Loaches who enjoy a reclusive life hidden in cave-like structures.

The loach would be marked in the red quadrant on the circle between the CALM and the RECLUSIVE  lebels. The Neon Tetra would be marked in the green quadrant between the SOCIAL and CALM labels on the circle.  So, these two fish would share the attribute of CALM and push into the two opposing areas of SOCIAL and RECLUSIVE.

Based on the behavior quadrant these two fish would be behaviorally compatible and could share the same aquarium.

If another occupant is desired, that occupant could be in the upper half of the behavioral quadrant and be fully compatible with the existing occupants of the aquarium.

Water flow is an environmental area that is difficult to plot an aquarium occupant against.  An occupant has different tolerances for water flow speed and volume depending upon age, health, and current behavior.  To make the process slightly less difficult, it is assumed that the water flow area will represent a healthy occupant as an adult exhibiting normal behavior.

Using this criteria it is possible to determine what water flow conditions the potential occupant is most comfortable with.  Generally, larger occupants are tolerant of greater water flow with greater volumes of water.

The majority of potential occupants will exist somewhere within the circle representing an aquarium fish that can maneuver within the turbulence generated by aquarium equipment.

The majority of potential occupants will exist somewhere within the circle representing an aquarium fish that can maneuver within the turbulence generated by aquarium equipment.

A compatible fish in this element pair would be a fish that closely mimics the existing preference for water flow.  The reason for this stems from the difficulty involved in creating a fast moving stream next to a stagnant piece of water – in the same aquarium.  Not that it can’t be accomplished with enough tank area and proper aquarium technology to move water.  It would just be more involved and require good hydro engineering.

Water flow is plotted against the cave dweller/open water elements.  An aquarium fish typically exists in an aquatic environment that provides for isolation and protection or open water swimming.  The isolationist rarely emerges from their protective cave.  The exceptions would be for feeding, mating, or defending.

The polar opposite to the protective cave is the open water swimmer.  Some fish prefer not to be caught in a cave and are more inclined to swim in the open area of the aquarium.

Many aquarium occupants find themselves somewhere within the circle on these elements.  A potential compatible occupant to an existing fish population would be one that prefers the polar opposite of the existing population. As an example if the aquarist has a loach that prefers to be hidden, the aquarist should be looking for a fish that prefers to open swim as a compatible fish.

Environment 2 expands upon Environment 1 by introducing Vegetation and Illumination element pairs.

The Vegetation element represents a range of aquatic fauna from sparsely placed plants to thick forests of vegetation that can be difficult to see through.  Some aquarium occupants requires thick fauna to hide in and to forage for food.  Others prefer environments more easily navigated with unobstructed views.

Generally, most occupants fall somewhere in the middle.  There are exceptions where some fish require hiding areas and tall grasses on which to deposit eggs.  This type of egg-laying behavior requires floor grass for breeding.

For compatibility purposes it is possible to have polar opposites in the same aquarium for this element.  If the aquarium is sufficiently large, having areas of dense vegetation can satisfy those fish needing this type of fauna.

Plotted against Vegetation is Illumination.  Illumination is a somewhat complicated element to navigate when it comes to compatibility.

The Illumination element scale is between 16 hours and 8 hours of illumination.  This is because most aquarium occupants come from aquatic regions that can vary between 16 and 8 hours of daylight.

This is not to say an aquarist couldn’t create a biome mimicking the northern hemisphere and southern hemisphere supporting lighting requirements consistent with the sunlight phenomenon know as the “midnight sun”.

The intensity of light should be appropriate for the types of fish and plants in the aquarium. Some fish species come from habitats with bright, direct sunlight, while others are adapted to more shaded conditions.

Consider the following factors when determining light intensity and duration:

Low-Light Species: Fish species that originate from shaded environments or waters with dense aquatic vegetation may not require high-intensity lighting. Low to moderate lighting is often sufficient for these fish.

High-Light Species: Certain species, especially those from clear, well-lit waters, may benefit from higher-intensity lighting. Brighter lighting can enhance the coloration and activity levels of these fish.

Plants: If you have live aquarium plants, their lighting requirements should be taken into account. Most aquatic plants require moderate to high-intensity lighting for photosynthesis and growth.

Color Spectrum: The color spectrum of the light can affect the appearance and behavior of fish. While fish can see a broader range of colors than humans, a white or daylight spectrum is generally recommended for providing a natural look and promoting healthy plant growth. Some aquarists use lighting with a slight blue or red spectrum to enhance the colors of certain fish species.

Vegetation and decor will act to buffer incompatibilities among occupants with varying illumination requirements.  Occupants that require higher amounts of illumination will not be wandering about the vegetation where the intensity of illumination would be less.  Occupants requiring less illumination will not be wandering away from the protective vegetation where illumination would be greater.

Compatibility in the illumination area among occupants will be dependent upon hiding spaces.  Without hiding spaces the graphing of compatible fish species will be the same among all occupants.  Compatibility among occupants in a mixed vegetation aquarium, where there are hiding places, will be best if there are both high and low intensity tolerant occupants. In this situation the compatible occupants would lean towards each polar side and not be clumped together.

Compatibility is a somewhat complicated topic.  The individual graphs are tools to aid the aquarist in making an informed decision regarding the introduction of new occupants to the aquarium.  It can be difficult to keep all the different conditions in one’s head regarding the existing aquarium occupants, and the potential new occupant’s requirements.

Use the graphs to understand your existing population, and to plan for new additions.