Test your ammonia 30-45 minutes after a feeding.. When you get it tested later on, you might of allowed a small ammonia sipke to cycle its way out of the tank..
I sometimes see people feed silversides and other seafoods very high in fats and oils, and seems to nasty up the water pretty quickly.. Like DW, water seems a bit cloudy.. Look up at the surface of your water, does it look like an oil slick? Do you get any bubbles or frothiness?
I agree with less additives and more small partial water changes.. salt and clean water is really a great way to help and save fish.
Like the others said.. that ray is very skinny, and if the other ray in the tank is putting on weight and the other is not, it's definately parasites. I am curious to wonder if the ray is having issues battling off these recently treated parasites internally, and is causing his internal organs massive amounts of stress..
ie: a belly full of worms just died from meds and now my body has to work its way out of there.. could be permenant damage to the inside of the fish? Who knows..
Keep up those small partials, heavy salt, and maybe think about offering a different type of food? I had success injecting parasite meds (metro) into nightcrawlers.. The rays which previously had white, stringy fecal matter, within days, would produce long solid curly feces.. That is a big sign of parasites or no parasites..
Have you sifted the poo lately?
