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Lydia,
I just lost a reticulated auratus to whatever you are describing. It turned from green and black to nasty brownish and black, and didnt move much. About 3 days later, it was dead.
I dont know what it was, and I dont have a vet around here that knows anything about frogs, so I ordered some powdered panacur. I assume it cant hurt to do a worming or two. I cant understand what else it could have been other than a parasite maybe. It was fat and healthy before, then just turned colors and got more and more lazy.
Guess i'd better update the signature. sigh.....
I feel your pain though! I'm sorry to hear that and if you find out what it was, let me know!