You might be able to keep a fish with an N. Wenn. I'd go with a damsel since they're super cheap, active, and aggressive. I've had one with my G. Chiragra for about a year with no harm done to the damsel or mantis. Almost any invert that's a part of your cleanup crew will get eaten, but if you feed your mantis frozen food (shrimp, squid, scallops, etc.) the cleanup crew will probably last longer. I end up buying new crabs/snails about every 1-2 months.