Hi guys, need some help. My stingray suddenly developed redness in its mouth and i see it opens its mouth widely when it climbs the glass or glides on water. It has been like this for a week now and it also stopped feeding. It is still quite active but probably because it is irritated / stressed. The other fish in the tank is a xb aro and another stingray. This is the only one that has redness on its mouth. The tank is ok with 0 ammonia and nitrite, ph of 7 while nitrates is at 80. So far I have done a 50 pct wc and put salt in the tank and I do not know what to do next. Thanks as usual for the help guys ....