I'm sorry, but you cannot keep a JD in a 20gal, they need at least a 45gal is you plan on just keeping that fish. A pacu is way out of the question, they need at least a 145gal if you plan on keeping only that fish. The only way you would be able to keep 4-6 cichlids in a 20gal, is if you got some type of new world dwarf cichlid, or some type of small african species.