sting ray

I don't know what to keep in a 150 gal tank. I like arowanas and alligator gars but I knwo they can not be housed in any aquarium because they would be i humane and cruel. I'd rather stay away from cichlids. I was think about freshwater rays or some other oddball fish. I would like some kind of...

1 of my ray has a sunken head. She is fat, eats like a pig and very active. The other rays are fine. My nitrates was a little high, could that be the cause? I did a water change and the it seems to be a little better.