That is what I would thing got to it. Something got in from that. All it takes is one the size of a dime or less and it could destroy everything. The pond has enough volume to disperse the concentration to the others but if the ray ingested it some how it would get taken out. Rays are also hyper sensitive to things like that, they even vary in specimens as to tolerance for different things. That one could have been more sensitive to whatever got in then the others and it killed it. The other two may have been a bit stressed but acted fine. The only way they have to clear their systems is to 'sweat it out' thus the slime coat. Much like big cats, but big cats revert to vomiting before anything else in most cases. My limas will shed slime coat if something is not right. They shed copious amounts also. Found this out with my bad prime experiences. Now I test all new dechlors on their tank first. That way I can see right away if something is not right before it gets into my aros or rays. The cats can handle it better and tell you right a way. I always leave enough of the last bottle to do a few changes on the trial tank if need be.
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