In a 20 gallon you would only be able to have one GSP. These can be very aggressive fish towards any other tank mates, not a good idea to have anything else in the tank with GSP. Go with crushed coral, it seemed to work better with my GSP and make sure you ask petstore what the salinity is in theire tank so you can get yours close to theirs. Hope you have done research on the salinity level increases as they grow. Full marine by adults.