Ok so now with the large fish I have in my tank between the algea scrubber and my protein skimmer I am at an untested able level for nitrates and phosphates. I know it will prolly go up to 1 or 2 once full grown and in the system but now I have perfect levelsWell - Coral Catsharks do best if they are kept in groups(3 or more). So imagine the bioload of 3 adult coral catsharks in 300-400 gallon pond.
You might be able to have a few other smaller fishes, or a small yellow/cortez round stingray or maybe a small eel - if you have a really good filtration system.