They are different breeds, the Ocellaris is sometimes called a False Percula because they look similar to the True Percula. True Percs are harder to breed in captivity so fish stores will almost always have wild caught ones for sale, while Ocellaris are really easy (with the right setup) to breed in captivity. Ocellaris are also hardier/easier to care for. True Percs get to about 4 - 5 inches in captivity while Ocellaris get to about 3.5 - 4.