Red leg is usually accompanied by sores on the legs they will spend a lot of time in there water, and they usually become very lethargic from what I’ve heard and read. I have never had a frog die of red leg so I can’t speak from first hand experience. If you were cleaning and disenfecting that often the red leg shouldn’t have came from your tank. If it was redleg I imagine he may have got it from the store, once they got it even if you take and do all the things to make them better they still end up passing most the time. Once they have it in my eyes it’s hard to get rid of, it’s very unforgiving. For the coconut fibre he could have became impacted but agin usually with impaction they wouldn’t die that fast I don’t think, he would have to become septic from not going poop. Or if it was really bad it could have tore the lining of the stomach or intestine and would have leaked out which would have killed him but I dont think is what happened. Coconut fibre is made and meant for if the frog ingests it that he can pass it. I think the frog more then likely was just I’ll or as I mentioned had very bad genetics.