Fish compatibility depends on four things together: adult size, temperament, water parameter needs, and available tank space. There's no single rule that covers every species pairing — check your specific fish against all four before combining them.
How do I know if two fish species are compatible?
Compatibility isn't one factor, it's the combination of several. Two peaceful species can still clash if one grows too large for the other to feel safe around, or if their water parameter needs are too different to share a tank long-term. Checking all four factors together, rather than just temperament or just size, catches problems a quick search often misses.
The four things to check
- Adult size — look up the full-grown size of every species, not just their size at purchase. Many popular fish are sold small but grow much larger.
- Temperament — semi-aggressive or territorial species often can't be mixed with peaceful, slow-moving ones long-term.
- Water parameters — species requiring very different pH, hardness, or temperature ranges are harder to keep together comfortably.
- Tank size and hiding spots — even compatible species can clash in a tank too small to give each one its own space; size for the most territorial species in the mix, not the average.
Compatibility at a glance
| Factor | What to check |
|---|---|
| Adult size | Full-grown size for every species, not purchase size |
| Temperament | Peaceful vs. semi-aggressive vs. territorial |
| Water parameters | Shared pH, hardness, and temperature range |
| Tank size | Sized for the most territorial species present |
Already seeing aggression between fish you've combined? See the fish aggression guide for how to fix it, or run the 2-minute fish diagnosis if something else seems off.