Fish Compatibility
Not every two fish belong in the same tank. Here's what actually decides it — instead of trusting a "peaceful" label on a store tag.
Key facts
- Temperament matters more than purchase size — check adult size, not the size the fish is sold at.
- Shoaling fish (tetras, corydoras and similar) need a group, not a pair — commonly recommended at 6 or more of the same species.
- Matching water parameters and matching temperament are two different questions: a pair can share one and still clash on the other.
What actually decides compatibility
There is no single universal compatibility chart — there is a set of factors worth checking for any specific pair of species.
- Temperament: peaceful, semi-aggressive or aggressive.
- Adult size — many fish sold small grow far larger than they look in the store.
- Water-parameter overlap: temperature, pH and hardness — a pair can get along temperamentally and still need different water.
- Swimming zone: top, mid or bottom of the tank — a balanced community stocks across zones rather than crowding one.
- Tank size relative to the fish — a species peaceful in a large tank can turn territorial in a small one.
- Activity level — fast, darting species can chronically stress slower, shyer tankmates.
Source for this framework: Charterhouse Aquatics, a fish-compatibility guide for home aquariums.
Shoaling fish need a group, not a pair
Many popular small aquarium fish — tetras, corydoras, danios and similar species — are naturally shoaling. Kept alone or in a pair, they show chronic stress, which affects both immune health and lifespan. Hobbyist consensus commonly sets a floor of around 6 of the same species for most shoaling fish, with some nano species recommended in even larger numbers.
This is general hobbyist guidance, not one scientific figure that applies to every species — check the recommendation for the specific fish before buying it.
Before you buy, check a species profile — not a chart
Exact temperament, water-parameter needs and adult size vary enough from species to species that any single universal compatibility chart is always a simplification. Before adding a new species to a stocked tank, check that species' own profile — SeriouslyFish is one of the most widely used independent fish species-profile references in the hobby — rather than relying on a general rule.