Rays produce a lot of waste, that's why you really need a lot of filtration, even with sustained water changes. Powerheads are usually the "polishing" the water items, not the filtration of a setup. Get yourself a good canister going on that tank (Rena xp4, FX5, or a couple Fluval 405s).
I calculate my filtration for "fish tanks" to be as close to 10x the return as possible, so for a ray tank you'd need to match that, or better (talking about tank, huge ponds is a different story).