While there are "simple" setups in saltwater, the owners still have to know what is going on in the tank, which is somewhat complicated and requires a little research. If you are willing to do this you'd be able to keep a clown or two in a ten gallon (although I think tomatoes get a little large, look into ocellaris or percula). You can't just put "sand and rock and plants" and expect success, especially in a 10 gallon. Study up and develop a plan before you dive in. Hope that helps...
John