with discus and 14 different stingrays living in his pond, im hesitant to say his water quality is that bad. if it were bad enough to start causing HITH in pbass, it should have killed off the rays and discus for sure by now i would imagine. id bet it has more to do with a poor diet.
i would try to broaden your fishes diet. prawn meat alone is most likely not the most ideal diet for them. try to get some pellets in there, or switch it up more with different fish fillets, shrimps, mussles, whatever you could find on sale at the grocery store that week.