Fish care calculator

Fish Compatibility & Tankmate Checker | FishZone

Use the Coexistence Lab to cross-reference temperaments, temperature bounds, pH ranges, and swimming depths of freshwater species before adding them to your aquarium. Prevent territorial disputes, physical injury, and chemical shock before you buy.

  • Scientific Formula
  • Reviewed by Experts
  • Updated May 2026
  • Instant Results

Type

Risk Assessment

Inputs

3

Difficulty

Beginner

Calculation Time

Instant

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Quick Answer

Aquarium species compatibility hinges on five core parameters. Water Chemistry: pH and temperature ranges must overlap — Discus require 82–86°F and pH 5.5–6.5, while Mollies need 72–78°F and pH 7.5–8.5, making them completely incompatible. Behavioral Temperament: peaceful, semi-aggressive, or aggressive — Tiger Barbs will relentlessly fin-nip slow, long-finned fish like Bettas and Guppies. Adult Size: if one fish can fit another into its mouth, it eventually will — a 6-inch Angelfish will consume Neon Tetras. Swimming Layer: surface, mid-water, and bottom zones should be distributed to avoid competition for space and food. Territoriality: cichlids require visual barriers, caves, and sometimes intentional overstocking to disperse aggression. Always introduce new fish through a 2-week quarantine tank, observe behaviour for the first 72 hours, and have a rehoming plan ready for persistent aggressors.

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Enter your details

Enter the labeled values below. Results appear without leaving this page.

How to Use This Calculator (Worked Example)

This tool requires 3 key inputs:

  1. Select Species A: Enter your specific value (e.g., from your tank's test kit or dimensions).
  2. Select Species B: Enter your specific value (e.g., from your tank's test kit or dimensions).
  3. Choose Species to Explore: Enter your specific value (e.g., from your tank's test kit or dimensions).

Example: If you input average baseline values, the calculator will immediately process the formula and return the recommended output and safety warnings above.

Method

This tool uses the visible inputs on the page and returns practical aquatics guidance for maintaining your tank.

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The Problem

Fishkeepers regularly purchase fish based on appearance or impulse without verifying whether new additions are compatible with their existing community. The result is avoidable injury, predation, chronic stress, disease, and livestock loss — all of which are emotionally distressing and financially costly. Compatibility information is scattered across forums, books, and species-specific care sheets, making it time-consuming to cross-reference multiple species simultaneously. This calculator centralises that verification into a single, instant check.

When to Use This Tool

Use this calculator before purchasing any new fish — especially when adding a species you have not kept before. Use it when building a stocking list for a new tank from scratch to verify all planned species are mutually compatible. Use it when troubleshooting unexplained aggression, hiding, fin damage, or stress in an existing community, as an overlooked incompatibility is frequently the underlying cause. Use it when advising other hobbyists or gift-buyers on safe species selections.

Who is this for?

  • Beginner aquarists planning their first community freshwater tank stocking list
  • Intermediate hobbyists adding new species to an established aquarium
  • Experienced fishkeepers researching biotope-accurate or species-specific setups
  • Cichlid keepers managing aggression in African or South American setups
  • Betta owners wanting to build a safe community around a centrepiece fish
  • Fish store customers who want to verify compatibility before purchasing
  • Parents and hobbyists setting up a first family aquarium and needing beginner-safe species combinations
  • Aquascapers selecting species that will not disrupt planted tank aesthetics or root systems

Building a peaceful community aquarium is one of the most rewarding challenges in the fishkeeping hobby — and one of the most common sources of heartbreak when it goes wrong. A fish lost to an incompatible tank mate, a beautiful specimen destroyed by fin-nipping, or an entire school wiped out by a water chemistry mismatch are all preventable with the right information before you buy. This calculator exists to give you that information.

Who Should Use This Calculator?

This tool is for any freshwater aquarium hobbyist planning a new community tank, adding new fish to an established setup, or troubleshooting aggression and stress in an existing aquarium. Beginners will use it to build their first safe community stocking list. Intermediate hobbyists will use it to verify that a new species they spotted at the fish store is actually compatible with what they already have at home. Advanced keepers managing species-specific biotope tanks or cichlid communities will use it to cross-reference water chemistry parameters with precision.

It is equally useful for fishkeepers researching a purchase online at midnight as it is for someone standing in a fish store trying to decide in the moment.

The Five Pillars of Compatibility

Successful cohabitation depends on all five of the following parameters aligning — not just one or two. A fish pair that matches on temperament but clashes on water chemistry will still fail.

1. Water Temperature: The acceptable temperature ranges of two species must overlap by at least 4°F (2°C) to form a viable long-term pairing. Keeping fish outside their preferred range causes chronic immune suppression, making them permanently susceptible to disease even if they are not visibly sick.

2. Water pH: pH ranges must overlap by at least 0.5 units. A mismatch causes physiological stress at the cellular level — affecting osmoregulation, enzyme function, and gill health — often before any visible symptoms appear. Chronic pH mismatch shortens fish lifespans significantly.

3. Temperament: Mixing hyper-aggressive or territorial fish with passive, slow-moving species almost always results in injury, chronic stress, or death. Temperament categories — peaceful, semi-aggressive, and aggressive — are starting points, not guarantees. Individual fish personalities vary, and a fish labelled peaceful in one setup may become a bully in an overcrowded or understimulated tank.

4. Adult Size: The golden rule of fishkeeping: if a fish can fit another into its mouth, it eventually will — even if both fish have lived peacefully together for months. Always research the adult size of every species, not the juvenile size you see in the store. A 1-inch juvenile Angelfish will reach 6 inches within a year.

5. Swimming Layer: Fish naturally occupy one of three vertical zones in the water column: surface, mid-water, or bottom. Concentrating too many species in a single zone creates competition for food, space, and resting areas, driving aggression even between otherwise peaceful species.

Zoning Your Aquarium

To minimise territorial disputes and feeding competition, stock fish that inhabit different vertical zones. Combining surface-dwellers (such as hatchetfish or African butterflyfish), mid-water schoolers (such as tetras and rasboras), and bottom-feeders (such as corydoras catfish and kuhli loaches) creates a balanced, dynamic display while keeping stress levels low. A useful target ratio for a community tank is approximately 30% surface species, 50% mid-water species, and 20% bottom-dwelling species by fish count.

When to Use This Calculator

Use it before purchasing any new fish — ideally while still at home with time to research, not standing in the fish store. Use it when planning a new tank stocking list from scratch to verify all species are compatible before buying any of them. Use it when you notice unexplained aggression, hiding, fin damage, or stress in an existing tank, as an incompatibility you missed may be the root cause. Use it to educate yourself on species before gifting fish or recommending them to other hobbyists.

Common Problems This Calculator Prevents

The most frequent compatibility mistakes in freshwater fishkeeping are: mixing Goldfish with tropical species (temperature and waste incompatibility), housing Neon Tetras with adult Angelfish (size predation), combining African and South American cichlids (water chemistry conflict), adding Tiger Barbs to a tank with Bettas or Guppies (fin-nipping aggression), and mixing multiple bottom-feeding species without adequate space and food distribution. Each of these mistakes is entirely avoidable with a compatibility check before purchase.

A Note on Quarantine

No compatibility check replaces a proper quarantine protocol. Always isolate new fish in a separate quarantine tank for a minimum of two weeks before introducing them to your main display. This prevents the introduction of parasites, bacterial infections, and viral diseases that can devastate an established community regardless of species compatibility. Observe initial interactions carefully for the first 72 hours after introduction, and always have a backup plan — a spare tank or a trusted local fish store willing to take back an aggressor — before adding any new species.

Reference Table

Static Fish Compatibility Calculator Reference

These baseline ratios and decision rules are included directly in the page so visitors can review core guidance before using the interactive calculator.

Scenario Baseline Rule Safety Note
Inputs used by this tool Select Species A, Select Species B, Choose Species to Explore The interactive calculator refines the result from these inputs.
Outputs generated Pair status badge, Pair status desc, Temp pct val, Temp summary text The static table gives baseline logic; final value depends on entered values.
Species PairingTemperature Compatible?pH Compatible?Temperament Match?Size Safe?Overall Verdict
Neon Tetra + CorydorasYes (72–78°F overlap)Yes (6.0–7.0 overlap)Yes (both peaceful)Yes (similar size)Excellent — classic beginner pairing
Betta + Harlequin RasboraYes (74–78°F overlap)Yes (6.5–7.0 overlap)Generally yes (rasboras are fast)Yes (rasboras too large to target)Good — with dense planting and 20+ gal
Angelfish + Neon TetraYes (75–78°F overlap)Yes (6.5–7.0 overlap)Risky — Angelfish are predatoryNo — Neons fit in adult Angelfish mouthAvoid — high predation risk at adulthood
Goldfish + Tropical communityNo — 10°F+ gapPartial overlap possibleGoldfish outcompete for foodRisky — Goldfish grow largeAvoid — temperature and bioload conflict
Mbuna Cichlid + DiscusPartial (78–80°F possible)No — pH 8.2 vs pH 5.5–6.5No — Mbuna are aggressiveRisky — Mbuna will harass DiscusAvoid — water chemistry conflict is critical
Tiger Barb + GuppyYes (74–78°F overlap)Yes (7.0 overlap)No — Tiger Barbs fin-nip constantlyRisky — Guppy fins trigger attacksAvoid — fin damage and chronic stress guaranteed
Corydoras + Kuhli LoachYes (72–78°F overlap)Yes (6.0–7.0 overlap)Yes (both peaceful bottom dwellers)Yes (similar size)Good — different microhabitats reduce competition
Oscar + any small fishYes (tropical range)Yes (neutral pH)No — Oscars are ambush predatorsNo — Oscars eat any fish under 4 inchesAvoid — predation inevitable as Oscar grows
Troubleshooting Guide
1 One fish is relentlessly chasing all tank mates

2 New fish are hiding and refusing to eat after introduction

3 Fish developing torn or ragged fins without visible aggressor

Glossary of Terms
Temperament
A classification of a fish species' typical behavioural disposition toward other fish. Categories include peaceful (unlikely to initiate aggression), semi-aggressive (may chase or nip but rarely causes lethal harm), and aggressive (regularly attacks and injures tank mates).
Swimming Layer
The vertical zone of the water column a fish naturally inhabits. The three layers are surface (top 2–3 inches), mid-water (central 60% of tank height), and bottom (substrate and lowest 20% of tank height). Distributing species across all three layers reduces competition.
Bioload
The total volume of waste produced by fish in an aquarium, which directly affects water quality through ammonia, nitrite, and nitrate accumulation. High-bioload species such as cichlids and goldfish require significantly more filtration than their body size implies.
Quarantine Tank
A separate, bare-bottom holding tank used to observe and treat new fish before introduction to the main display aquarium. A minimum 2–4 week quarantine prevents the spread of parasites, bacteria, and viruses to established livestock.
Fin Rot
A bacterial infection characterised by fraying, ragged, or disintegrating fin edges, sometimes with a white border at the damage zone. Caused by opportunistic bacteria (commonly Aeromonas or Pseudomonas) that exploit fish weakened by poor water quality, physical injury, or stress.
Drip Acclimation
A slow acclimation method in which aquarium water is gradually siphoned into the bag or container holding a new fish at a rate of 2–4 drops per second over 45–60 minutes. This minimises osmotic and pH shock during the transition between the source water and display tank water.
Dither Fish
Small, active, fast-moving schooling fish added to a tank to reduce the anxiety of shy or reclusive species. Their confident swimming signals to cautious fish that the environment is safe. Common dither fish include Danios and Rainbowfish.
Territorial Reset
The practice of rearranging aquascape elements — rocks, driftwood, and plants — before introducing new fish. Disrupting established territory boundaries prevents resident fish from treating the entire tank as claimed space when newcomers arrive.
Osmoregulation
The physiological process by which fish maintain a stable internal balance of water and dissolved salts (ions) regardless of the external water chemistry. Fish living outside their preferred pH or salinity range must work harder to osmoregulate, diverting energy from immune function and growth.
Labyrinth Organ
A supplementary breathing organ found in Anabantid fish (Bettas, Gouramis, Paradise Fish) that allows them to breathe atmospheric air directly. Fish with a labyrinth organ are highly territorial around the water surface and will aggressively compete with other labyrinth fish for surface access.
Schooling Fish
Species that instinctively form tight social groups for safety, navigation, and stress reduction. Schooling fish kept in groups below their minimum number (typically six) experience chronic stress, reduced immunity, and often increased aggression toward tank mates as a displacement behaviour.
pH Shock
A rapid, acute stress response caused by a sudden change in water pH, typically defined as a shift of more than 0.3 pH units per hour. Symptoms include erratic swimming, loss of equilibrium, and in severe cases, death within hours. pH shock is preventable through slow drip acclimation.
Scientific References
  1. Fishbase — Global Species Database for Freshwater and Marine Fish
  2. Baensch Aquarium Atlas Volume 1
  3. The Simple Guide to Freshwater Aquariums

Frequently Asked Questions

Can I put these two fish together in the same tank?
That depends on five things: whether their temperature ranges overlap by at least 4°F (2°C), whether their pH ranges share at least 0.5 units in common, whether their temperaments are compatible, whether neither fish is large enough to eat the other, and whether they occupy different swimming zones in the water column. If all five align, cohabitation is generally safe. If any one of them does not, you risk stress, injury, or loss — even if the fish appear fine initially. Use the compatibility checker above to cross-reference any two freshwater species instantly.
What are the exact temperature and pH overlaps needed for two freshwater fish to live together?
For a safe long-term pairing, temperature ranges must overlap by at least 4°F (2°C) and pH ranges must share at least 0.5 units. For example, Neon Tetras (68–78°F, pH 6.0–7.0) and Discus (82–86°F, pH 5.5–6.5) have zero temperature overlap and should never be kept together. By contrast, Harlequin Rasboras (72–78°F, pH 6.0–7.0) and Corydoras Catfish (72–78°F, pH 6.0–7.5) overlap fully on both parameters and make excellent tank mates. When in doubt, a 45–60 minute drip acclimation helps buffer the transition, but it cannot compensate for a fundamental parameter mismatch.
What fish are compatible with a Betta fish in a community tank?
Male Bettas can coexist with peaceful, non-nippy species that avoid the surface layer and do not have long flowing fins. Safe companions include Corydoras catfish (all species), Kuhli loaches, Otocinclus catfish, Harlequin rasboras, Ember tetras in schools of eight or more, and Nerite or Mystery snails. Avoid all gouramis (they share the labyrinth organ and trigger territorial responses), Tiger Barbs (fin-nippers), Guppies and Endlers (their tails trigger attack behaviour), and any fish significantly larger than the Betta. A minimum 20-gallon (75-litre) tank with dense planting and at least three caves is recommended. Always add the Betta last so it cannot establish territory before its tank mates arrive.
Can Goldfish live with tropical fish?
No — this is one of the most common and damaging compatibility mistakes in the hobby. Goldfish thrive at 65–72°F (18–22°C) and produce significantly more ammonia than most fish their size due to their digestive physiology. Tropical fish require 75–82°F (24–28°C) and are far more sensitive to ammonia spikes. Even if you compromise at 74°F, both species are living outside their comfortable range, chronically stressing their immune systems. Keep Goldfish in species-only or cold-water community setups with White Cloud Mountain Minnows, Weather Loaches, or Hillstream Loaches instead.
Will Angelfish eat Neon Tetras?
Yes, reliably once the Angelfish reaches adult size. Juvenile Angelfish and Neon Tetras can sometimes coexist peacefully, which misleads many hobbyists into thinking the pairing works. However, an Angelfish grows to 6 inches (15 cm) in body length and views any fish under approximately 1.5 inches (4 cm) as prey. Neon Tetras are a natural food source for wild Angelfish in the Amazon basin. If you want a tetra with Angelfish, choose larger species such as Congo Tetras, Bleeding Heart Tetras, or Black Skirt Tetras, which are too large to swallow.
Which fish are the most common fin-nippers and what should I avoid keeping with them?
The most reliably identified fin-nippers in the freshwater hobby are Tiger Barbs, Serpae Tetras, Buenos Aires Tetras, Black Widow Tetras, and to a lesser extent, Denison Barbs and some Danio species when underschooled. Avoid keeping these species with any fish that has long, flowing fins — Bettas, Fancy Guppies, Angelfish, Discus, and Fancy Goldfish are all high-risk targets. Fin-nipping is typically a schooling behavioural outlet: keeping Tiger Barbs in groups of eight or more focuses intra-species interaction and significantly reduces nipping directed at other species.
Can African cichlids live with South American cichlids?
No. African Rift Lake cichlids such as Mbuna, Peacocks, and Haplochromines require hard, alkaline water at pH 8.0–8.6 and temperatures of 76–82°F (24–28°C). South American cichlids such as Discus, Angelfish, Geophagus, and Apistos prefer soft, acidic water at pH 6.0–7.5. There is no stable water parameter compromise that serves both groups well simultaneously. Beyond chemistry, aggression levels differ dramatically — Mbuna are among the most aggressive freshwater fish kept in the hobby, while many South American cichlids are comparatively peaceful. Mixing the two continents leads to deaths, typically within two weeks.
How do I stock a cichlid tank to reduce aggression?
The most effective strategy for African Mbuna cichlids is intentional overstocking combined with heavy decor. Aim for 15–20 fish in a 75-gallon (284-litre) tank, maintain a ratio of one male to three or four females per species, and provide at least two cave structures per fish using stacked rock or ceramic tubes. Rearranging the aquascape monthly resets established territories, preventing any single fish from becoming dominant over a fixed area. For South American species such as Geophagus or Festivum, the opposite applies — understocking reduces competition and allows subdominant fish space to escape and recover.
What does 'semi-aggressive' actually mean and can semi-aggressive fish live in a community tank?
Semi-aggressive means the species displays territorial chasing, occasional fin-nipping, or food guarding but does not cause lethal violence under normal conditions. Examples include Dwarf Gouramis, Rainbow Sharks, Clown Loaches, and juvenile Angelfish. Semi-aggressive fish can absolutely thrive in community tanks when paired with robust, similarly sized peaceful species such as larger tetras, Rainbowfish, or Geophagus cichlids. They should not be housed with small, fragile, or slow-moving fish. Providing ample visual barriers (driftwood, tall plants, rock formations) and feeding multiple small portions daily — rather than one large feeding — reduces the competition that drives most semi-aggressive behaviour.
How does swimming layer affect fish compatibility?
Fish naturally claim specific vertical zones in the water column. Surface dwellers such as Hatchetfish and African Butterflyfish occupy the top 2 inches and need calm water and floating plants. Mid-water species such as Tetras, Rasboras, and Angelfish need open swimming lanes through the central 60% of the tank. Bottom dwellers such as Corydoras, Loaches, and Plecos require sand or fine gravel substrate and sinking foods that reach them before mid-water fish consume everything. When multiple species compete for the same zone, aggression and stress increase even among otherwise peaceful species. A practical target is roughly 30% surface, 50% mid-water, and 20% bottom by fish count, with hardscape and tall plants creating natural separation between zones.
Why do my fish seem compatible at first but start fighting after a few weeks?
Several factors can cause delayed aggression. Juvenile fish rarely display their full adult territorial behaviours, so a peaceful pairing in the first weeks may turn hostile as fish mature. Hormone changes as fish approach breeding condition significantly increase aggression in many species, particularly cichlids and gouramis. An established fish that tolerated a newcomer initially may become assertive once it has had time to map and claim territory. Overcrowding stress compounds over time as fish grow. If aggression develops after a honeymoon period, review stocking density, add more visual barriers, separate the aggressor temporarily to reset its territory status, and ensure all fish are receiving adequate food.
What is the mouth-size rule and why does it matter?
The mouth-size rule is straightforward: if a fish is large enough to fit another fish entirely into its mouth, it will eventually eat it — regardless of how long the two have coexisted peacefully. This is an instinctive predatory behaviour, not a learned one, and it is not influenced by how well-fed the larger fish is. The risk increases at night when lights are off and smaller fish are less alert. Always research the adult body length — not the juvenile size you see in a store — of every species in your tank. As a conservative guide, no fish in a community tank should be smaller than one-third the adult body length of the largest predatory species present.
Can I keep multiple male fish of the same species in one tank?
It depends entirely on the species. Multiple male Bettas in the same tank will fight to serious injury or death — no exceptions. Multiple male Guppies coexist peacefully in adequate numbers. Male African cichlids can be kept together in heavily stocked, appropriately sized tanks with multiple females to redirect territorial energy. Male Dwarf Gouramis will typically fight when housed together in tanks under 40 gallons. The key variable is whether the species is solitary and highly territorial by nature or whether it forms natural social hierarchies in the wild. Research the natural social structure of each species specifically rather than applying a universal rule.
How do I introduce new fish to an established community tank without triggering aggression?
Quarantine new fish for at least two weeks in a separate tank before introduction. When ready to add them, rearrange your existing aquascape the day before — moving rocks, driftwood, and plants resets established territories so resident fish are too preoccupied with re-mapping their space to focus on newcomers. Add new fish after lights-out, when territorial behaviour is reduced. Feed the tank immediately after introduction to further distract residents. Monitor closely for the first 72 hours. If a resident fish is relentlessly targeting the newcomer, have a separator divider ready to use. Never introduce a single new fish into a large existing group of schooling species — add at least three to six individuals so attention is distributed.
What are the most incompatible freshwater fish pairings and why do they fail?
The six most reliably problematic pairings are: Goldfish with any tropical fish (temperature and ammonia incompatibility); Neon Tetras with adult Angelfish (size predation — tetras are natural prey); male Betta with male Betta (lethal territorial fighting); Tiger Barbs with Guppies or Bettas (relentless fin-nipping causing stress and fin rot); African Rift Lake cichlids with South American cichlids (incompatible water chemistry and aggression levels); and Common Plecos with other bottom dwellers in undersized tanks (adults reach 15–24 inches and become territorial, occasionally attaching to and damaging the slime coats of catfish and slow-moving fish).
How many fish can I put in my tank — what is a safe stocking level?
The traditional guideline of one inch of fish per gallon is oversimplified and frequently misapplied. A better approach considers bioload (how much waste a fish produces relative to its size), swimming activity level, and territorial space requirements. A single Oscar cichlid at 14 inches produces more waste than twenty 1-inch Neon Tetras. More practical guidelines: for peaceful community fish, aim for one inch of adult fish per 2–3 gallons in a well-filtered, planted tank; for aggressive or territorial species, calculate each individual's space requirements separately. Always account for adult size, and always understock relative to your filter capacity — a cycled, mature filter running at 5–10 times tank volume per hour can support moderate stocking sustainably.
Do I need a quarantine tank and how do I set one up?
Yes. A quarantine tank is one of the most important pieces of equipment any fishkeeper can own, yet it is consistently skipped by beginners. A 10–20 gallon bare-bottom tank with a sponge filter (seeded from your main tank), a heater, and a simple hide structure is all that is required. New fish should be quarantined for a minimum of two weeks — four weeks is better — before introduction to your display tank. This allows you to observe and treat any parasitic, bacterial, or fungal infections before they spread to your entire community. Ich, Velvet, and internal parasites frequently arrive on new fish showing no visible symptoms in the first few days.
What fish are good for a beginner community tank?
The most forgiving and reliably peaceful beginner community fish for a 20-gallon or larger setup are: Neon or Cardinal Tetras (school of six to eight), Corydoras catfish (school of six, any species), Harlequin or Lambchop Rasboras (school of six to eight), Cherry Barbs (not the fin-nipping species), Otocinclus catfish (algae cleaners, group of three to five), and a single male Dwarf Gourami as a centrepiece. All of these species share compatible water parameters (72–78°F, pH 6.5–7.5), peaceful temperaments, and different swimming layers. This combination is low-aggression, visually diverse, and appropriate for a moderately filtered, planted tank.
Why is my fish hiding all the time after I added new tank mates?
Persistent hiding after a new introduction almost always indicates stress caused by intimidation or harassment — even if you do not observe direct aggression. Fish hide when they feel unsafe, and the trigger is often subtle: a larger fish making repeated passes, competition at feeding time, or being excluded from a favoured resting spot. Check whether the hiding fish is being chased during feeding, has any fin damage or missing scales, and whether it is eating at all. If hiding persists beyond 48–72 hours, separate the suspected aggressor and observe whether the hiding behaviour resolves. Long-term hiding causes immune suppression and frequently precedes disease.
Can I mix saltwater and freshwater species in the same tank?
No. Freshwater and marine (saltwater) fish have fundamentally different osmoregulatory physiology. Freshwater fish maintain higher internal salt concentrations than their environment and continuously absorb water through osmosis. Marine fish do the opposite — they drink seawater and excrete salt. Placing either in the wrong salinity causes rapid osmotic shock and death within hours. Brackish water species such as Mollies, Archerfish, and Figure-Eight Puffer Fish occupy the middle ground and can tolerate a range of salinities, but they are not compatible with true freshwater or full marine species.