The goby shrimp pairs are great. I've had mine for over a year and a half. Yes, the do move around a lot but in a 20, they are usually visible. Green banded goby are also pretty cool. I recently added on to my tank to go with the shrimp/goby and a bi color blennie. They are very social and will come out to see you when you go near the tank. I cleaned some in the tank today and he followed my hand around in the tank the entire time.