I hope your ray feels better soon. I agree that he may have had a tummy ache. I also feed earthworms from Walmart and Bruce has gotten a tummy ache from them once. Just try to make sure they are really really clean, and maybe feed only worms one day then mix the other foods on the other days.
Also I would like to say that it is NOT always the water. People who assume everything is the water need to realize that yes they live in water, but they are still animals with many working organs in them.
I never did figure out what happend to Bruce that almost killed him (he was in death curl for over 24 hrs yet pulled through) It was not his water, and have no clue what happened. I think it was internal.