Personally, I'd go with one of the smaller mantises, such as G. smithii, G. ternatensis, N. curacaoensis, or P. ciliata. I have a peacock in my 20g with a few softies, some macro and an orangetail blue damsel (I would only advise mixing with fish if you're certain you have a "smasher" type, which typically prey upon mobile inverts more than fish, and go with a fish that will stay in the water column more, so no gobies, blennies, dragonets etc.). I love him though, beautiful and intelligent, everyone that comes over wants to ask about him. However, if I could do it over again, I would have waited longer to get my hands on one of the smaller species I listed above, since I'm almost positive he'll need a bigger tank eventually. Another great option would be a pistol shrimp & goby pair. You could even splurge a bit on one of the pricier ones, since there wouldn't be any competition for food or aggressors in the tank. If I were in your shoes and I had some extra cash I'd look into the dracula or maybe a white cap if you can find one. Some other great options are the yasha and the classic hi fin red banded goby. Good luck :beer: