How to choose compatible aquarium tank mates
AquaLog guide · Published
In short: Fish make good tank mates when their water needs overlap, their temperaments match, none can eat or bully the others, and each species has the group size and space it needs.
1. Do their water needs overlap?
Every fish has a temperature, pH and hardness range. Pick species whose ranges overlap comfortably, not just at the edges. Neon tetras (pH 5.5–7.5, soft water) and mollies (pH 7.5–8.5, hard water) barely meet. Cold-water fancy goldfish should not share a tank with tropical fish at all.
2. Do their temperaments match?
Keep peaceful fish with peaceful fish. Semi-aggressive species such as tiger barbs do well with other robust, fast fish, but they nip the fins of slow, long-finned fish. That is why our care guide lists the betta, angelfish and guppy among the fish to avoid with tiger barbs.
3. Can one fit in the other's mouth?
Plan for adult sizes, not the size in the shop. A fish that can swallow another eventually will. Angelfish grow large enough to eat small tetras, which is why neon tetras list angelfish among the fish to avoid.
4. Do they use different parts of the tank?
Spreading fish across the top, middle and bottom of the tank reduces competition and makes a fuller tank. Bottom-dwellers such as bronze corydoras pair well with mid-water schoolers. Every species page in the care guide shows where that fish swims.
5. Can you keep enough of each?
Schooling fish feel safe only in a group; kept alone or in pairs they hide, lose colour or turn nippy. Neon tetras need at least 8, harlequin rasboras 8, and bronze corydoras 6. Make sure the tank has room for every group at full adult size.
How do you introduce new fish?
- Add peaceful and smaller species first, and boisterous ones last.
- Rearranging decor before adding newcomers resets territories.
- Watch the first few days for chasing, torn fins or fish refusing food, and have a plan if it doesn't work out.
- Add a few fish at a time so the filter keeps up. See how to set up your first freshwater aquarium.
Every species in the AquaLog care guide lists good tank mates and fish to avoid. Signed in, the "Will it fit your tank?" check compares a species with the tanks you've logged: size, water type and your latest temperature and pH. Try it with your tanks.