What can I add to my fish tank?

I have a 20 gallon fish tank, I have tropical fish already and YES the tank is heated. I have 2 fiddler crabs, 1 red wag platy, and 1 zebra danio. I like colorful fish that move alot. What can I put with these fish? Keep in mind, I have fiddler crabs so they have to be able to be with them.