What Fish Can Live Together? A Beginner's Guide to Tankmates

This is the line you can never cross: freshwater fish and saltwater fish cannot share a tank. They are built for completely different water and putting them together kills one side fast. Once you are inside the right water type, the details still matter. Temperature is the big one. A goldfish wants cool water while a betta wants it warm, so the two make poor roommates even though both are freshwater. pH and hardness come next. Soft, acidic blackwater fish like many tetras are happiest in conditions that rift-lake cichlids would hate. When two fish want the same temperature and similar pH, you have cleared the first hurdle. When they do not, no amount of good temperament will make the pairing work.

Every fish sits somewhere on a scale from peaceful to aggressive. Peaceful community fish mind their own business. Semi-aggressive fish are mostly fine but will defend a territory or chase at feeding time. Aggressive fish treat the whole tank as theirs. The trick is to keep your stock inside one band. A peaceful schooling fish housed with an aggressive tankmate lives in constant stress even if it never gets bitten, and stressed fish get sick. If you love a feisty species, build the rest of the tank around fish that can hold their own at a similar size. If you want a calm community, keep everyone in the peaceful range and the tank mostly runs itself.

Almost every fish you see for sale is a juvenile. The inch-long common pleco stuck to the glass today grows past a foot and a half and needs a tank to match. A bala shark looks like a tidy little community fish at three inches and turns into a foot-long shoaling fish that needs a six-foot tank. The single rule that saves the most fish is this: if a tankmate is small enough to fit inside another fish's mouth, sooner or later it will end up there. Judge a pairing by the sizes these fish reach as adults. The babies in the store tank tell you almost nothing. A fish that grows to two inches is a snack for one that grows to eight, no matter how peaceful that bigger fish is on paper.

Fish naturally split a tank into three layers. Hatchetfish and many livebearers cruise the top. The middle belongs to most tetras and barbs, with rainbowfish darting through. Down on the floor you find the catfish and the loaches, and plecos working the same ground. Stocking across all three layers does two useful things. It makes the tank look full and active at every level, and it cuts down on squabbles because fish are not all crammed into the same water fighting for the same space. If you put six bottom-dwellers in a tank and nothing else, they compete with each other for territory. Mix in some mid-water schoolers and a couple of top swimmers and everyone has somewhere to be.

A few patterns cause most of the fin damage in home tanks. Known fin-nippers such as tiger barbs and serpae tetras will harass slow fish that trail long flowing fins, the kind you see on a betta or an angelfish or a fancy guppy. Keep nippers away from anything that trails fins. Schooling fish bought in twos and threes get nervous and turn nippy, while the same species in a group of six or more relaxes and behaves, so buy schoolers in proper numbers. Small shrimp and tiny snails round out the list. They are wonderful cleanup crew in a peaceful tank, but they read as food to anything with a big enough mouth, so only pair them with small, gentle fish.

Not every fish wants company, and there is nothing wrong with a tank built for one species. African clawed frogs eat anything they can catch and are best kept alone or with their own kind. Many large cichlids, some pufferfish, and a number of predatory species are the same way. A single-species setup is often the most relaxed tank in the house because nobody is being chased and nobody is being eaten. If a fish you love comes with a reputation for living solo, believe it and give it a tank of its own rather than fighting its nature.

Every livestock listing on our site shows the tankmates we would actually keep with that fish, with a plain reason for each pairing drawn from published care sources, plus the fish's adult size and temperament and the minimum tank it needs. Start there to build a shortlist that already fits together. Then call us at (814) 456-9445 or stop by the shop in Erie and tell us your tank size and what you already have. We will sanity-check the plan against your specific water and stocking before you spend a cent, which is a lot cheaper than learning the hard way.