Some dense carbon filters, like those from Berkey, may require a post-filter to remove fluoride and can introduce aluminum oxide contamination.

“oh they don't it's very hard to get fluoride out of your water even a berky filter will not get fluoride out of the water unless you use their post filter which then puts aluminum hydroxide into the water or aluminum oxide it so you're going to so the berky won't even get FL it's very hard to filter fluoride out”