What decides whether two fish can share a tank
Seven things, and none of them is whether the fish look nice together.
- Adult size. Not shop size. Almost everything in a shop tank is a juvenile, and a great many conflicts only appear when one of them finishes growing.
- Whether one fits in the other's mouth. This is the one that ends worst and it is entirely predictable from two numbers.
- Temperature. A tank can only be one temperature. If the two ranges barely overlap, one fish spends its life at the edge of what it tolerates.
- pH and hardness. Less critical than the hobby once believed — stability matters more than hitting a target — but soft blackwater species and hard-water livebearers really do want opposite ends of the scale.
- Fins. Some fish nip, some fish trail long fins, and putting the two together is the most common avoidable failure in a community tank.
- Temperament. Aggressive, semi-aggressive, peaceful — and a large semi-aggressive fish bullies a small peaceful one at feeding time even when nothing gets eaten.
- Invertebrates. Shrimp and snails are food to a surprising number of fish that are otherwise perfectly peaceful.
Where the hobby genuinely disagrees
Fish compatibility is contested, and not in a trivial way. Two people who have each kept serpae tetras for a decade will tell you opposite things about them, and both will be describing what they saw. The same argument runs about bettas in community tanks, about how many gallons an oscar needs, and about whether mollies want salt.
So the rules behind this page are deliberately conservative, and where a species is genuinely argued about, the note printed under the verdict says so rather than picking a side. A caution here means this is the pairing that goes wrong most often, not this is banned.
What a bigger tank fixes, and what it does not
Length and broken sight lines defuse a lot of territorial aggression. A pair that fails in a 30-gallon tank can genuinely work in a six-foot one, because the loser has somewhere to be.
Nothing fixes a size difference and nothing fixes temperature. A tank cannot be two temperatures at once, and a fish that fits in a mouth still fits in it at the far end of a hundred gallons.
The two figures shops understate
Adult size and minimum tank length. A common pleco is sold at three inches and reaches eighteen. A bala shark is sold at three and reaches twelve. A clown loach is sold at two, reaches a foot, and lives past twenty years. None of this is a secret, and none of it is usually on the label.
A yes here is not a stocking plan
Two fish being compatible says nothing about whether your tank can carry them. Bioload, filtration turnover, water surface area and footprint all still apply, as do schooling minimums — most of the fish on this page are miserable in twos and threes and need six or more of their own kind. Schooling minimums covers that, and the full calculator checks the whole tank.
Print the sizes and minimums
The table below is the part people most need at the shop counter: what each of these fish actually grows to, the smallest tank it belongs in, and how many of its own kind it needs.
| Fish | Adult size | Min tank | Min length | Min group |
|---|