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You have several options here. The biggest problem you will face with a preditor tank is controlling water parameters. 125gal may seem like a lot of realestate to work with, but you will need to stock very lightly in order to maintain acceptable water parameters. I would stay away from SW
rays. A 125 is far too small for them. They need lots of room and an unobstructed sand bottom. Lions, tangs, triggers, puffers are all good options. What are you using for filtration?