I use a carbon block filter and an alumina filter. Alumina gets out the fluorine. I’ll have to check on what the carbon block removes on the magnesium
Did you use trans dermal magnesium? Good for your husband! This guy is very good on magnesium and iodine http://iodine.imva.info/
We use magnesium malate. The RDA for mg is 425mg. We use double that.
I have the transdermal mag and use it from time to time but it’s a wee bit sticky (like after swimming in the ocean).