FE (Iron) - The FE is for water with ferrous iron. Ferrous iron is iron that has not been oxidized. This is where you run water in a glass and it is clear but over time turns yellowish green.
RE (Sulfur) - Hydrogen sulfide is a gas easily identified by its “rotten egg” smell. This is caused by decaying matter in your water. It is more commonly found in well water than municipal water. The AquaOx by itself will eliminate a lot of the rotten egg smell but if you take a shower and someone opens the front door and can smell the odor you might want this add on.
Kills and removes viruses and bacteria from your water.