Fish for a 24 gallon

[Whatigot]

you could get a neon goby...
They're parasite eaters and pretty tiny and don't need substrate to be happy and are also very colorful.
Also, try a clown goby, they are by far my favourite nano fish and are very cheap compared to many others and also don't require substrate, not to mention there are several awesome colors and they are very entertaining.
I would say you're 50/50 in a tank that small with having the royal gramma turning mr. t on everyone else.
Maybe an orchid dottyback instead?