You might consider leaving the Epsom salts in the tank with fish I use a tsp. per gallon to start and increase it to 1 TBL per gallon, leaving the Epsom Salts in the tank. I haven't kept a Clawed Frog though, but leaving it in the tank may help. I have not
Do you have a filter and is it cycled?
I would do 100% water changes, to make sure there is not any ammonia in the water to further stress him.