Get a really cheap
microscope and take some of the poo and mash a slide of it.
If you're not sure how to identify the source of the problem, check out this
simple google-search found identification primer, find your own, or purchase software/books on the identification of stingray parasites / infections. Think about it- stingrays are generally pretty expensive. Isn't it worth it to protect them, rather than shotgunning them with medication?
Step 1) Identify the disease (either through tell-tale signs, such as
visible external parasites or other symptoms, or find out using a microscope)
(Note: Only after completing step 1)
Step 2) Decide on a treatment
Step 3) Administer treatment, observe, repeat if necessary
(Note: This method works for educators, raykeepers and researchers. I know that this isn't always obvious for importers, exporters or wholesalers, much less LFS stores, which is why they may give you their "shotgun" style advice. For the private keeper I urge you to consider the risks, both financial and emotional, of these.)
READ THIS:
http://www.monsterfishkeepers.com/forums/showthread.php?t=222954