Oscar and flowerhorn are not what I would call mediunm-sized. To me, both will get a little large for a 75 gallon, except for a shortbody varient flowerhorn. Between the two, I would go with FH over the Oscar for that tank size. Even better would be a fish that stays in the 10" range, like a Macaw cichlid Nicaraguense. A group of smaller cichlids like amatitlania would make a nicer display imo, since you'd get to watch the fish interact. But if you really want a single fish, nothing wrong with that either.
