One thing I read on a few websites is that isopods actually compete with nematodes for food and isopods usually win. So if you would have an abundance of isopods you will be able to choke out nematodes in theory. I use native plants I buy from nurseries and things that may have nematodes but I have an abundance of Springtails and Isopods that have successfully prevented my toads or anything else native from getting infected. For the future idk if you do this already but you can attempt keeping isopods with your frogs. I'm unsure of a survival rate of frogs from your question. Some vets will inject the frog directly as my vet did to a dumpy frog with an airborne infection and it helped the frog. He didn't feel it. The Pedyalite will work the one you suggested it gives energy. I've used it before. Sugar added to warm water helps also. Helps them poop too.

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