if tank is too warm frogs can suffer from fungal infections of the skin
check the tank for warmth and if you are going to replace the taddies etc.
best to wash it before putting anything new in there
fungus is sometimes not noticeable on the frog but it coats the skin
a frogs skin is porous and can allow sprays, toxins etc to be absorbed into the frogs metabolism, so don't clean tank with any bleach or disinfectant
boiling water and a good scrub is best
sorry for your experience with the frogs etc