Fish tank mate compatibility: what fish can live together

Curated by the Fast Aquatics editorial team · Updated May 2026 · Sources

Most failed community tanks fail because of incompatible fish, not water quality. Here's the framework + the no-go pairings hobbyists keep relearning the hard way.

The 4-factor compatibility model

Before adding any fish to your tank, check 4 factors against your existing stock:

Adult size match. Largest fish in tank shouldn't be more than 2x the smallest. Small fish are food for big fish.
Temperament. Peaceful + peaceful = OK. Peaceful + semi-aggressive = caution. Peaceful + aggressive = no.
Water parameters. Soft acidic + hard alkaline = no. Cool water + warm water = no.
Activity level / niche. Both fish need their own space. Top swimmers + mid swimmers + bottom dwellers stack better than 3 mid-level schoolers.

Classic incompatible pairings

Goldfish + tropical fish. Different temps (60-72F vs 76-82F).

Tetras + barbs. Tiger barbs fin-nip slow tetras to death.

Bettas + fancy guppies. Bettas attack the guppy fins.

Discus + most cichlids. Different parameters + temperament.

Multiple yellow tangs. Conspecific aggression.

Neon tetras + angelfish (adult). Adult angels eat neons.

Mbuna + peaceful community. Mbuna are aggressive open-water cichlids - destroys community fish.

Reliable community combinations

Top + middle + bottom layered FW. Hatchetfish (top) + tetras (middle) + corydoras (bottom). Each owns its niche.

Reef-safe community. Ocellaris clown + royal gramma + firefish + 6-line wrasse + cleanup crew. All peaceful, different niches.

Planted-tank community. 1 male betta + 6 ember tetras + 5 amano shrimp + 3 nerite snails. Top + mid + bottom + cleanup.

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Frequently asked questions

Are clownfish compatible with tangs?

Yes. Clownfish (peaceful) and tangs (semi-aggressive but reef-safe) coexist fine in 75+ gallon reef tanks. Add the clownfish first, then add the tang to a tank that's been stocked for 30+ days.

Can I keep two female bettas?

Sometimes - in a 'sorority' of 5+ females in a 30+ gallon planted tank with broken sight lines. Less than 5 = bullying. Two = fight. Skip the sorority unless you have the right setup.

What fish are compatible with goldfish?

White Cloud Mountain Minnows, Weather Loaches, Hillstream Loaches, and other cool-water fish. NOT tropical species like tetras or guppies (temp incompatibility). Goldfish-specific tank is usually best.