A 1-gallon fish tank is a small tank option and is unsuitable for most fish. There are a few options available that will suit a 1-gallon tank, though there are limitations on the number of fish appropriate for the small tank size. In general, most fish species would only allow a single fish in the tank at any given time.
Guppies are a small fish that are social. Since the fish are social, a small tank is not appropriate because only one fish can be in the tank, which would socially isolate the fish. For the short term, such as when a guppy is dropping fry, a 1-gallon tank with plenty of plants is appropriate because it allows the new fry somewhere to hide, and separates the pregnant guppy to prevent the adults from eating the fry at birth.
Betta fish can tolerate a 1-gallon fish tank. Since betta fish are both aggressive and small, keeping a single fish in a 1-gallon tank is appropriate. For betta fish, the 1-gallon tank is only appropriate if there is a single fish. For more than one fish, the tank is too small and the fish will attack each other.
White clouds are an appropriate fish for a 1-gallon tank because they are very small fish, usually only about 1-inch long when fully mature. The small size allows them to fit into the tank without difficulty. Owners must limit the number, usually to one or two fish at the most, due to the small size of the tank.
Fish Selection Rule
In general, a 1-gallon tank is too small for fish because the general rule for fish tank size is that a fish needs 1 gallon of water per 1 inch in length. Any fish that grows larger than 1 inch is too big to fit in the tank comfortably. Generally, the best fish for a 1-gallon tank is a betta because they are hearty and need separation from other fish.