I would use prazi for wild caught rays like that. It clears the parasites well and doesn't harm them as long as you follow the directions. I also think the water change may have been the culprit. How much was the change? There is something to be said about how much fish can get used to dirty water conditions. Sometimes when you change it quickly, the large variations in pH or nitrates can actually shock the fish and cause a problem. Also if you have city water you might want to ask around to see if anyone else lost a fish that day. I have heard of problems in the area where they use a chemical to flush the lines that causes fish death especially for sensitive species, but is fine for humans.
I do 50% changes weekly on a 240 gallon with 6 rays. I haven't had any problems for the two years I have had them now, even got some pups on the way. Big changes often is the trick here, plus look at your filtration, if you think you have enough, double it. I have bio filtration rated for 1600 gallons now, despite feeding 8 ounces of silversides or shrimp daily I never have a biological load issue.
