The four things that decide compatibility
1. Water type & parameters
This is the dealbreaker most beginners miss. A fish bred for soft, acidic water (most tetras, rams) will slowly fail in the hard, alkaline water African cichlids need — and vice versa. Match temperature, pH range, and hardness first; nothing else matters if the water's wrong.
2. Adult size
That cute 1-inch fish at the store is a juvenile. Check the adult size and the tank it needs. A common pleco hits 18 inches; a bala shark needs a 6-foot tank. If a tankmate can fit another fish in its mouth when both are grown, assume it eventually will.
3. Temperament
Peaceful, semi-aggressive, and aggressive fish mix badly. Fin-nippers (tiger barbs, some tetras) shred long-finned fish like bettas and angelfish. Territorial fish need space and sight breaks. Schooling fish kept in too-small groups get stressed and nippy themselves — keep them in groups of 6+.
4. Bioload & stocking level
Every fish adds waste. Overstock and ammonia and nitrate climb faster than your filter and water changes can keep up — which stresses everyone and invites disease. Compatible fish in an overstocked tank still fail. Leave headroom.
Quick compatibility red flags
- Mixing a known fin-nipper with any long-finned or slow fish.
- One fish that needs hard/alkaline water and another that needs soft/acidic.
- A predator whose mouth is bigger than a tankmate's body.
- Two males of a territorial species in a tank with no room to escape.
- Adding anything to a tank that isn't fully cycled yet.
Check it in seconds — before you buy
Standing at the store tank trying to remember all this is where mistakes happen. TankMate's compatibility scanner lets you snap a photo of any fish or coral and checks it against your tank — size, water type, temperament, and what already lives in it — then tells you compatible, caution, or not recommended with the reason. No guessing from memory at the counter.