My tank has been running for 4 months and is currently only housing a CUC, Softies and a cleaner shrimp! It’s a 20G long. I plan to get a pair of O clowns in the next few weeks, I would also like to add a green clown goby. I could probably get away with one more fish in there so what should I add? I’m thinking maybe a royal gramma or a talbot damsel. I don’t want firefish because I don’t trust them with my lid! What else would you suggest is an option for my 4th fish? Has to be coral & invert safe