I have in my 24 gallon:
1 Clarki Clownfish (Kind of a jerk, I'd probably not get him if I could go back and do it again, maybe another type of clown is more peaceful).
1 Blue Bar PseudoChromis (Awesome, likes hanging out in the rocks, very peaceful for a Dottyback).
1 Assesor (You could also get a Royal Gramma if you wanted, just as colorful).
1 Yellow Watchman Goby (You could get another burrowing sand sifter, like the yasha haze)
Another fish is the firefish or purple firefish. If you substituted that for a clown you would have a very colorful and peaceful tank.
Or a cardinalfish, some of those are very cool.
Smaller Gobies are harder to keep since they have picky diets (The Court Jester Goby & Mandarin are the 2 worst to keep).
Just do as much reserch as you can.
www.marinecenter.com is another good place to learn about fish.
Just don't add them all at once
