Duanes' post is what I would do; cryptoheros species. Or thoricthys. You could have a small group in your tank size and it will be much more interesting than a single larger fish in my opinion. The wet pet thing is great if the fish is super owner-interactive, but if the fish likes to just cruise around or is scared of the owner, I find little enjoyment that kind of setup. I also think a severum would get a little large for your tank, I have owned an 8"+ individual and have seen them over 10"