Being that niger triggers aren't as aggressive as other trigger fish, you have a wide variety of fish to select from. It does somewhat depend on the size of your tank, but I have seen a few niggers even kept peacefully in a reef tank. You could always throw in a few hermits and snails and see how he reacts. Or, you could try a bigger diamond goby and see how that does. You could even put a very engineer gobies into the tank. These are all possibilities that you can play with. I don't know how aggressive your niger is to other tankmates, so it is a hard question to answer.