Is it possible to get your water tested? Given your water change schedule we need to rule out ammonia poisoning as the culprit--you should take some of your old water to the pet store and get it tested for free to make sure your schedule is really as efficient as you think it is. Any detectable level ammonia is detrimental to your fish, so keep that in mind.
Also, not all internal parasites manifest themselves in the GI tract--if your fish is wasting away, he could have internal parasites that live in the muscle tissues of the fish. Although intestinal worms are more common, he could feasibly have an infestation in other tissues. If he's still eating and wasting away, it may be prudent to use a medication that includes praziquantel and metronidazole to help flush out any nasties that he might be carrying. Deworming medication is a lot less harsh on your fish's health than other treatments like malachite green or antibiotics, so keep that in mind as well. Methylene blue is another treatment that might be a good idea to help keep him from succumbing to secondary infections in his vulnerable state--it is a much more effective preventative than aquarium salt and gentle and quite helpful for dealing with ammonia poisoning.
In my experience, I have found that crowntail rays are a very good reflection of their general health--chances are if the rays are thin, brittle, and breaking, something is going on that should be remedied, be it water quality, sickness, temperature, etc.
Last edited by Adastra; 10-11-2010 at 02:52 PM.