Do some homework buddy, cuz there are a million different answers to that question, maybe if you got an axelrod's atlas and flipped through it may narrow down the field. There are lots of small fish.
I know there's alot Of small fish I just wanted peoples opinion about it. I was thinking of something more exotic besides just like some sort of minnow, goldfish, tetra
I keep Betta strohi "Ninja Betta" and a stunted BN pleco (Ancistrus sp.) in my 10 gallon. The pleco may be a bad idea but the Betta strohi is a good idea and lovely fish.. Not my pic below but a nice example from bro Google.
scarlet badis badis is nother great idea that is not done often but very pretty fish. Another bro Google example: