The fish store will sell you whatever you point at. The fish will figure out the compatibility problems for you, usually overnight, usually in a way that involves missing fins or missing fish.
Stocking a community tank well isn't complicated once you understand the factors involved. It's mostly just research — but you have to do the research before you buy, not after.
Water parameters first
This is the compatibility check most beginners skip. Every fish has a preferred temperature range, pH range, and hardness range. Keeping a fish outside its preferred parameters isn't immediately lethal — most fish are adaptable — but it causes chronic stress that shortens their lifespan and makes them vulnerable to disease.
The problem is that different regions of the world produce water with very different chemistry. African cichlids from the Rift Valley prefer hard, alkaline water (pH 7.5–8.5, GH 10–20). Discus from the Amazon need soft, acidic water (pH 5.5–6.5, GH 1–4). You can't keep them together, and you shouldn't try to find a "compromise" water that makes both unhappy.
Pick a biotope — a water type — and stock fish that come from similar conditions. South American fish (tetras, corydoras, most plecos, angelfish) generally prefer soft, slightly acidic water. African cichlids want hard and alkaline. Asian fish (rasboras, most barbs, loaches) are usually adaptable and work well in community setups.
Size and the "will it fit in my mouth" rule
Fish eat other fish. This is not aggression — it's feeding behavior. If a fish can fit another fish in its mouth, it will eventually try to eat it. A full-grown angelfish will eat neon tetras, not because it's mean but because neon tetras are food-sized.
Research the adult size of every fish you're considering, not the juvenile size. The cute 2-inch fish at the store is often a juvenile that will reach 8–10 inches. Common Plecos (Hypostomus plecostomus) are sold as algae eaters for small tanks but grow to 18–24 inches. Oscar cichlids are sold as 3-inch juveniles but become 12–14 inch aggressive fish that eat everything smaller than themselves.
Temperament and territory
Some fish are peaceful, some are aggressive, some are fin nippers, some are territorial during breeding. The general categories:
Peaceful community fish — most tetras, rasboras, danios, corydoras, otocinclus, most livebearers. These work with almost anything that won't eat them.
Mildly aggressive or nippy — tiger barbs, serpae tetras, some rainbow fish. These work in a community tank but can stress long-finned fish like bettas or angelfish by nipping their fins.
Semi-aggressive — most cichlids, larger gouramis, some loaches. These often work in a community tank at the right size but have specific compatibility requirements. A pair of keyhole cichlids is fine in a community tank; a pair of convict cichlids will kill everything else when they breed.
Aggressive / predatory — oscars, large catfish, most large cichlids, arowanas. These are species tanks, not community fish.
The stocking order matters
Add fish in order from least aggressive to most aggressive. If you add the dominant fish first, it establishes territory over the entire tank. Adding timid fish afterward means they're entering territory that's already claimed. Add the timid fish first, let them establish comfort zones, then add more assertive species.
Add fish in groups, not singles. A schooling fish like a neon tetra kept alone is stressed. Most tetras, rasboras, and barbs need groups of 6 or more to behave normally. A lone tiger barb picks on other fish; six tiger barbs mostly pick on each other.
A few combinations that sound good but don't work
- Bettas with other bettas — Male bettas fight to the death. One per tank.
- Goldfish with tropical fish — Different temperature requirements, and goldfish produce enormous amounts of waste that overwhelm most filters sized for tropical fish.
- African cichlids with community fish — The cichlids will eat or terrorize everything else.
- Plecos with slow, long-finned fish — Common plecos will rasp the slime coat off sleeping fish. Happens overnight, looks like a disease.