Most lfs sell Scats. I thought of them because they are elusive, hardy, greedy eaters and I have kept them with all sorts of aggressive fish. Their are several types available. My favorite is the most common "the green scat". They are good looking fish too. Another fish that comes to mind is a clown loach. Just google them or search in this site. Other things to consider is the size of your tank. If the tank is to small, the fish will be in each other's territories. In that case you may be better of getting rid of one instead of adding more.