I'm sorry for your loss.

I wish I could have helped you, but my knowledge on diseases is mostly with your typical fishes. I don't know much about exotic pets like stingrays, eels, etc. What may have happened is he/she did a lot of damage to the epidermis which removes the slime coat needed to protect the fish from bacteria. I'm not sure though.

Again, I'm sorry for your loss.
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Phishie

"An ounce of prevention is worth a pound of cure."