Free tool
Can these two fish live together?
Pick any two of 807 species. We check water parameters, predation risk, fin nipping, temperament and tank size — then draw both fish at life size so you can see the difference for yourself.
How to read the result
The checker returns one of three verdicts, and always shows its working. A good match means no measurable rule was broken. Possible with care means something needs managing — a narrow parameter overlap, a size gap that is not yet dangerous, or a temperament that depends on the individual. Not recommended means a hard conflict: no shared water parameters, or one fish is large enough to eat the other.
The single most useful line is usually the size one. Most fish that vanish overnight were not bullied, they were eaten — a fish will generally take anything under about a third of its own length. Two entirely peaceful species with identical water requirements are still a bad pairing if one reaches 30 cm and the other 4 cm.
No compatibility tool can promise an outcome, and you should distrust any that implies it can. Individual temperament varies enormously, especially in cichlids and bettas, and tank size, layout, planting and the order you add fish all change the result. What a checker can do is rule out the pairings that were never going to work and tell you exactly which factor to watch on the ones that might. That is what this one does.
Then check the whole tank
Two fish being compatible is not the same as a stocking plan working. Once you have a pairing you like, put the full list through the stocking calculator to check bioload, filtration and the combined volume, and read the setup guide for your tank size — 30 litre, 50 litre, 80 litre, 125 litre or 180 litre.
Track your parameters and get care reminders with App-aquatic.
Get the free appHow does the fish compatibility checker work?
Pick two species and the checker compares them on the things that actually decide the outcome: whether their temperature and pH ranges overlap, whether one is large enough to eat the other, whether either is a known fin nipper, how their temperaments interact, which part of the tank each occupies, and the combined tank volume they need. Each result explains which rule produced it, so you can weigh it against your own tank rather than trusting a colour.
Are the fish really shown at life size?
On a desktop screen, yes, within the accuracy your monitor allows. Browsers assume a fixed 96 pixels per inch, which is close but not exact on most displays, so the checker lets you calibrate against a bank card - every card is 85.6 mm wide by international standard. Once calibrated, the drawing is accurate to your specific screen and the setting is remembered. On phones there is not enough width for a large fish, so the view scales down and states the exact scale it is using.
Why does size matter so much for compatibility?
Because the most common cause of a fish disappearing overnight is not aggression, it is predation. A fish will generally eat anything that fits in its mouth, which is roughly anything under a third of its own length. Two peaceful species with identical water requirements are still a bad pairing if one is 30 cm and the other is 4 cm. Seeing the difference drawn to scale makes that obvious in a way a number does not.
Does a green result mean the pairing is guaranteed to work?
No. It means none of the measurable rules were broken - the water parameters overlap, neither fish can eat the other, and no known behavioural flags fired. Individual temperament still varies, particularly among cichlids and bettas, and tank layout, planting and stocking order all matter. Treat a green result as a sound starting point, not a promise.
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