you dont count cuc when stocking your tank, their bioload is extremely small, and since they eat other matter that would become ammonnia and turn it into pellets that can be easily vacuumed out with your maintenance they are a non-issue.
6 of even the smallest of fish would be overstocking a 20gal.
is your filtrataion system of the tank, as is the sand, they will help your water to stay pristine by having more in there... not to say youhave so much your crowding your fish, more is best. you can have a successful tank if you follow proper maintenance without any lr
, but you'd still top out at about 2-3 fish, such as 2 clowns and goby or blenny.