A better lion to put in there would be any of the species of dwarf lions.
A 75g is technically the minimum according to some sources, but most aggressive keepers agree they do much, MUCH better in a 100+g tank, especially if there are any other fish in the tank with him. Lions get large fairly quick, and a large tank is needed to keep tank levels and waste fairly stable. Lions are prone to bacterias and popeye when in a small tank with flucuating levels.