It's DEFINITELY a small sunfish... like a blue gill...
In Florida we call them Brim...
They sneak in as "contaminates" with the feeder fish or Ghost shrimp deliveries to your local fish store...
They're fun... Not too aggressive but will eat small fish...
Get a nice 2-1/2" - 3" Rhino Pleco...
They're nice looking, interesting, do a GREAT job on the algae, and when they get 5"+ they're worth something...
Won't bother the Calvus none...