If the verdict is caution advised, can I still keep them together?

Yes, if you change the conditions the cautions point at: a larger tank, more cover and visual barriers, a considered introduction order with the peaceful fish in first and the dominant one later, and a schooling group topped up to the recommended number. Caution advised means the risk is manageable with those changes, not that a disaster is inevitable.

Can I keep two of the same species?

It depends on the species. For a schooling fish such as a tetra, a Corydoras or a loach, two is the worst possible number, because the aggression has nowhere to spread and one fish takes all of it, so the group should be six or more. A territorial species such as a cichlid, a Betta or most marine fish will often fight until one dies when kept as a pair of the same sex, so either keep a single specimen or keep a group with enough space for individual territories.

Why does the checker set a minimum group size for schooling fish?

Because group size determines how safe the fish feel and how widely aggression is spread. Below the minimum the fish stop schooling and switch to hiding, lose colour and refuse food, staying in a chronic stress state. At or above it, the chasing inside the group is distributed across several fish and the pressure on any one of them falls. That makes the minimum group size a welfare and survival figure rather than a style preference.

Does this check replace quarantine?

No, the two answer unrelated questions. A compatibility verdict covers whether two fish will harm each other, while quarantine covers whether a new fish is carrying a pathogen. Even a compatible verdict does not mean the new arrival is clean, so it still goes through isolation and observation before it enters the display tank. Ich, velvet and Brooklynella all arrive in a reef tank that way.

Why are plants and corals not included in the checker?

Because their conflicts follow different mechanisms. Fish are limited by temperament and territory, corals by physical distance through sweeper tentacles and by chemical warfare through released toxins, and plants by light and CO2 tiers. Forcing all three through the same pairwise comparison would produce answers that sound plausible and are wrong. Their compatibility information sits on the individual group pages instead, as light, flow, aggression and CO2 requirements.

G-05 · 69 entries

Tank Mate Compatibility Checker: Pick Two Fish and See If They Can Share a Tank

This checker does one job: it surfaces the conflicts that objectively exist between two species. It does not judge how a pair looks and it does not predict individual temperament. Every input comes from data already published in the atlas, the water parameter ranges, adult sizes, temperament ratings and the small set of documented pairings, so you can check the verdict line by line. All of it runs in the browser, nothing about your parameters is uploaded, and the result page can be shared as a link.

Compatibility is not a question of whether two fish get along in general, but of whether one hard conflict exists, because a single hard conflict outweighs every softer note.

Pick two fish to get a verdict and the reasoning behind it.

How to use it

Step 1Pick two fishBoth dropdowns are grouped by freshwater and marine. Crossing the two is allowed, and the verdict will simply say why it is impossible.
Step 2Read the verdict, then the reasonsThe verdict is one of three: compatible, caution advised or not recommended. The reasons are ordered hard conflict, then caution, then note, with the heaviest first.
Step 3Check the recommended tank against yoursIt takes the larger of the two minimum tank volumes, then rounds up for schooling species. It is a floor, not a recommendation.

Frequently asked questions

If the verdict is caution advised, can I still keep them together?: Yes, if you change the conditions the cautions point at: a larger tank, more cover and visual barriers, a considered introduction order with the peaceful fish in first and the dominant one later, and a schooling group topped up to the recommended number. Caution advised means the risk is manageable with those changes, not that a disaster is inevitable.
Can I keep two of the same species?: It depends on the species. For a schooling fish such as a tetra, a Corydoras or a loach, two is the worst possible number, because the aggression has nowhere to spread and one fish takes all of it, so the group should be six or more. A territorial species such as a cichlid, a Betta or most marine fish will often fight until one dies when kept as a pair of the same sex, so either keep a single specimen or keep a group with enough space for individual territories.
Why does the checker set a minimum group size for schooling fish?: Because group size determines how safe the fish feel and how widely aggression is spread. Below the minimum the fish stop schooling and switch to hiding, lose colour and refuse food, staying in a chronic stress state. At or above it, the chasing inside the group is distributed across several fish and the pressure on any one of them falls. That makes the minimum group size a welfare and survival figure rather than a style preference.
Does this check replace quarantine?: No, the two answer unrelated questions. A compatibility verdict covers whether two fish will harm each other, while quarantine covers whether a new fish is carrying a pathogen. Even a compatible verdict does not mean the new arrival is clean, so it still goes through isolation and observation before it enters the display tank. Ich, velvet and Brooklynella all arrive in a reef tank that way.
Why are plants and corals not included in the checker?: Because their conflicts follow different mechanisms. Fish are limited by temperament and territory, corals by physical distance through sweeper tentacles and by chemical warfare through released toxins, and plants by light and CO2 tiers. Forcing all three through the same pairwise comparison would produce answers that sound plausible and are wrong. Their compatibility information sits on the individual group pages instead, as light, flow, aggression and CO2 requirements.

How it is calculated

Water parameter window: Temperature, pH and hardness for freshwater, or specific gravity for marine, are intersected between the two species. No overlap at all is a hard conflict. An overlap that exists but is too narrow, under 2 °C of temperature or under 0.4 of pH, is a caution, because there is almost no margin for temperature or chemistry drift.
Size ratio: The smaller adult length divided by the larger. If the larger species is predatory or aggressive and the ratio is under 0.45 it is a hard conflict, because the smaller fish becomes food, and under 0.7 it is a caution. Current size is never used, only adult size.
Temperament pairing: Two predatory or aggressive species of similar size, a ratio between 0.7 and 1.43, is a hard conflict from sustained fighting. An aggressive species paired with a peaceful one is a caution for fin nipping, and two semi-aggressive species are a caution for territorial disputes.
Same species and same genus: Two fish of the same schooling species get a note recommending a minimum group size. The same territorial species is a caution, since two males are the most likely pairing to fight. Two species in one genus, with at least one of them aggressive, is a caution for territorial conflict and possible hybridisation.
Logged pairings: A pair recorded in the atlas as a known conflict is a hard conflict outright, and a pair recorded as a proven combination produces a note. These entries override the general rules above, which is how the cases a rule would get wrong are covered.
Recommended tank: The larger of the two minimum tank volumes, raised by 30 percent when both species are schooling and by 15 percent when only one is. Tank length is the larger of the two requirements, because swimming room cannot be traded for water volume.