i found this info for you:
One person asked about salt and ADF's. Some people say to avoid salt entirely but I use about a tablespoon of aquarium salt per 5 gallons, and my ADF's are fine. Frogs are more sensitive to salt than some fish, perhaps as sensitive as some of the scaleless fish. That means, you do not want a lot of salt but some is okay. I would not put in more than a tablespoon per 4 gallons or so. If you do not have fish with the ADF's, do not use any salt. It is not needed for the frogs. I use it to help the fish in with them as my water is super soft and low in anions, cations, and minerals. The salt helps with the osmotic regulation of cations for the fish (sorry, I am a chemist!) and reduces the stress on the fish. This information on salt probably holds true for all other amphibians as well! A little is fine; a lot is bad. Too much salt will make water try to leave the frog and can leave him without enough water inside the body despite being surrounded by water. I also use low levels of salt in my ponds which have green frogs, bullfrogs, and other frogs in them. No problems have arisen.
here is her site
Robyn's Aquarium Frogs Page
hope this helps