What size tank can you put koi in I have a 30 gallon tank and want to get some koi?

That depends on if you fit the fish to the tank, or fit the tank to the fish. Koi will grow to the size of the tank. The bigger the tank, the bigger they will grow. You can put about 6 4 inch Koi in a 30 gallon tank. And of course, the bigger the fish, you need to adjust your tank size. The koi will quickly outgrow your tank! They will be like prisoners in a cell, not able to move around. Probably enough to survive if you keep up the waterchanges, but not enough to thrive. Koi belong in a pond.