Is the tank cycled? What type of filter do you have? Is it tropical or cool water? Whats your ph and hardness? If it is cool water and the ph is anywhere between 6 and 8, white cloud mountain minnows are good. If its tropical and the ph is anywhere between 6 and 8 you could try zebra danios, leopard danios, cherry barbs, black skirt tetras, etc etc etc.