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Personally, I would be more worried about figuring our WHY it died. As far as "chemicals" go...not realy anything to worry about. Dead and decaying animals can produe more NH3 in the water then your tank's biological filtration can process so the risk to additional livestock is still there. However, if you promptly removed the ray and have a good amount of biologial filtration in the tank you should be okay. Best practice would be to have plenty of prepared and aged SW on hand to do a water change if needed. Test water parameters (NH3, NO2 and NO3) twice a day for a week to make sure all is well. Turn your thoughts toward the casue of the death as well.
I agree Lando. I think they are worried about toxins being released (like a cowfish might do, if stressed or dead), as opposed to an ammonia spike, great advice on the checking of water parameters. I am unfamiliar with that ray so I can't really help you. If anything, you can run GAC to help remove any toxins, if you feel they may be present.