1. some sharks will suit a 20 gal fine, such as the rainbow shark, since they do not grow very fast
2. red tail sharks will become WAY to aggressive for a 20 gal community tank
3. columbian sharks like their water quite salty (we've had entire shipments of them die becaus someone put them in a fresh water tank)
4. and who the hell buys fish from walmart? in my opinion buying from walmart, petsmart or superpet are the lowest quality fish, also the people that work there don't know a damn thing about fish. at my work half the new customers are there becasue walmart or superpet has told them something stupid that cost them money.
and finally you probably shouldnt add anymore fish, or you may run into problems with stress or ammonia/nitrites/nitrates