I don't think I would want to eat aquarium fish. Alot of the chemicals used for water maintenance, and medications are absorbed into fish. Some even will accumulate and stay within the fish, and they are known to be toxic or cause cancer in humans. Not to mention most aquarium fish are riddled with parasites unless they are being treated with a dewormer every couple of months.
I'm sure you can eat them, and have little to no problems. But for myself, I wouldn't want to. Especially with seeing how polluted fish wild-caught are these days...and aquarium fish must deal with chemicals on a more regular basis just for general maintenance.
EDITED TO ADD: In South America Oscars are prized for their tastiness, they are suppose to be similar to Tilapia in taste.