If Iodine and mag are able to absorb into the skin via baths you better believe Fluoride is getting in too. I'm in the same quandary. Up until now there hasn't been a good solution short of reverse osmosis. There's a new point of use bone char filter on the market that might solve the bath problem sorta off.
My husband just bought a bone char filter water filter jug for work from the company. We've written to find out more about the water filter but they've not responded back. I'll get the company website from him and post here. I need to call them and get to the bottom of whether this will work for bath water.