The compatibility factors that matter most

Water chemistry requirements eliminate many would-be combinations before anything else. African cichlids require hard, alkaline water (pH 7.8-8.5, hardness 10-20 dGH). Most tropical community fish (tetras, rasboras, corydoras) prefer softer, slightly acidic water (pH 6.5-7.2). Trying to meet both requirements simultaneously produces water that is adequate for neither.

Temperature is the second filter. Goldfish thrive at 65-72 degrees F. Most tropical fish need 74-80 degrees F. Keeping them together means either the goldfish overheat or the tropicals are too cold -- chronic temperature stress suppresses immune function and shortens lifespans in both.

The peaceful community tank: species that reliably coexist

The most reliable peaceful community combination for a 20-gallon tank: a school of 8-10 small tetras or rasboras in the middle water column, 6 corydoras catfish on the bottom, and a centerpiece fish such as a single dwarf gourami or a small group of platies in the upper-middle zone. This covers all three swimming levels, requires similar water parameters (pH 6.8-7.4, temp 74-78 degrees F), and all species are too small to threaten each other.

Specific safe combinations: neon tetras + harlequin rasboras + corydoras (all prefer soft, warm, slightly acidic water); guppies + mollies + platies + cory catfish (all tolerate slightly alkaline, harder water); zebra danios + white cloud minnows + otocinclus catfish (cooler water, 68-74 degrees F).

Size mismatches: the rule fish keepers learn the hard way

Any fish large enough to fit another fish in its mouth will eventually eat it. This applies even to "peaceful" species. A 6-inch angelfish is peaceful toward fish it cannot swallow and predatory toward neon tetras (1.5 inches). A bala shark, sold as a community fish at 2 inches, reaches 12-14 inches and eats every small fish in the tank. Research the adult size of every fish before purchase -- the size at the pet store is juvenile size, not adult size.

Cichlids: the compatibility asterisk

Cichlids are among the most popular aquarium fish and among the most misunderstood regarding compatibility. Most cichlids are territorial and aggressive, especially during breeding. South American cichlids vary more: rams and apistogramma species are small (2-3 inches) and can coexist with peaceful community fish in larger tanks. Oscars, Jack Dempseys, and green terrors are aggressive and should be kept in species-only or cichlid-only tanks. African cichlids require other African cichlids of similar size -- a Lake Malawi cichlid tank works when stocked with multiple species at similar size to diffuse aggression through numbers.

Territorial species: one per tank or provide space

Betta fish (males): one per tank, no exceptions for males. Two male bettas in the same tank fight until one or both die. Betta females can sometimes be kept in groups of 5+ in a sufficiently large tank (20 gallons minimum) with many hiding spots -- this is called a sorority tank and requires monitoring for aggression. Male bettas can coexist with peaceful, non-fin-nipping tankmates: corydoras, snails, certain tetras and rasboras, and shrimp (in tanks without aggressive bettas).

Dwarf gouramis: males are territorial with each other in small tanks. In a 30+ gallon tank with visual barriers (plants, driftwood), two males sometimes coexist. In tanks under 20 gallons, keep only one male gourami.

Fast reference: incompatible combinations to avoid

Goldfish + tropical fish (temperature mismatch). Bettas + fin-nipping tetras like serpae tetras or tiger barbs (the nippy fish destroy betta fins). African cichlids + small community fish (African cichlids eat them). Any large predatory fish (oscars, arowana, large catfish) + small fish. Aggressive species (convict cichlids, red devil cichlids) + peaceful species. Saltwater fish + freshwater fish (obviously, but beginners sometimes ask).