fish decision help

[diavids]

I have a 20 gal reef tank that has been set up for 5 1/2 months. things seem to be doing well so far. I have one firefish and would like to add another fish but i don't know what. I would like a fish that is more outgoing than the firefish who comes out only at feeding and spends the rest of the time in or near his home. The fish used to be more outgoing but i have a hitchhiker crab who has taken up in the same rockwork. (i'm trying to caught the crab) So what would make a good companion fish for my firefish. I don't want any agressive fish please and not a goby who again would stay in the rockwork. I would love to add a shrimp but until i can get the crab out i woudn't want to try it. I'm thinking a common clown, but shouldn't they be kept in pairs? thanks for any advice.