srsly... are you medicating with it or just adding it cuz "you are suposed to"
umm.. anyway... google says 1 teaspoon per 10 gallons is safe for the frogs.. more than that and the might live but its reccomended to move them.... that may verywell be wrong.. just tellin ya what i found
yes they are fine with small amounts of salt but dont go crazy on that chemical crap for the fish because you can kill or harm the frogs...remember frogs can obsorb and breath through their skin...i have african clawed frogs but same maintenance except feeding wise, my biggest one is 8 inches lol