That might be part of the problem I don't think an fx5 is eneuff as rays are messy eaters and the spounges are likely harbouring a lot of detritus. I would up my filter look into a sump
How big are your Rays because an fx-5 in all aspects isn't enough for 2 good size rays. Not to be rude I'm just going off personal experience. You would need more overall turnover and more bio and mech filtration. I have an fx-5 on a 110 gal and it's not exactly what if consider up to my standards. But don't get me wrong it's a good canister filter.
And the op initially states he has an fx-5 and sump. The water he describes is pretty well perfect so.... Does it have parasites lice/flukes? Breathing heavy? Acting as if its sneezing from time to time?
Isn't the fx5 is rated for 700 g? I've had it running by itself for 2 years and never had a problem. Water params are always good. I change 50-70 gallons a week. Never a spike. I also have one power head