what kind of fish can be kept in a one gallon tank?

No FISH can be kept in one gallon tanks how ever a pair of dwarf shrimp would do well in there.(you'd need a heater of course)

I don't think I can name one animal that can be kept in 1 gallon tank.

Most shrimp would need like at least 1.5 gal with heating and filtration, preferably much bigger.

Bettas need 5 gallons with heating and filtration, light is not heating.

If thats an undergravel or a sponge filter maybe brine shrimp, nothing else really comes to mind.

No fish. Maybe you could keep a mystery snail, but even that would be a stretch.

1 gallon is too small to support any fish. It could be a temporary home for a Betta, but he would need to be moved into a proper tank very soon.

You could grow some beautiful plants and have a nice aquagarden. Maybe even some snails for some active lifeform in there.

Or you could do brine shrimp, triops, (aka seamonkeys and aquasaurs) or other little inverts too.

No fish available inthe aquarium market would work, even the hardy and resiliant betta dies young and tortured in such an environment. :(

I'm sorry but a one gallon tank is too small to house any fish. It is too small to heat safely, and is an unstable environment.

None that you like and want to survive. I guess if there was a fish that you really hated you could stuff it in there and torture it to death. But there are easier ways to kill fish.

Ian

A betta will do great in a one gallon tank. They are very colorful and don't need much work.