The first is an inline "fluoride" cartridge spliced into your fridge's water line, usually around $100 to $120. It filters one dispenser, it's often sold on a claim with no published test data behind it, and it does nothing for the water you cook with or fill the kettle from. The second is a multi-stage under-sink system with an activated alumina stage, which filters every glass at the kitchen tap and handles chlorine, lead, and PFAS in the same pass, with the fluoride reduction documented in its performance data rather than on the box.