I could use some help choosing fish species to put in a tank this small it hasn't been set up yet so most options are available though I might put some guppies from my 20 gallon in there once its setup
Harlequin or lambchop rasboras
Celestial pearl danios neon tetras, I think green are smallest. Ottos. Only a few and Will need an established tank. Pea puffers. popular oddball fish for a small tank.
Badis badis, pair. Killifish. Short lived but I believe happy in small tanks. Pygmy corys possibly depending on tank footprint.
Just gonna throw in shrimp tank as an option ? Again will need an established tank but some glorious colour to choose from. Just research first as with anything to see if it’s right for you or what you want ( my favourites are the yellow neos !)