I started with the 3 stage under the sink unit. Then I added this single filter to my refrigerator and hooked it up inside the cabinet with a shut off valve there, just in case, and so I don't need to move the fridge at all when it comes time to change that filter. and another single stage under my bathroom sink where I have in inline water flosser. And then the fourth and final under my kitchen sink AFTER my 3 stage waterdrop. I wanted to make sure that ALL fluoride is removed there and I DID NOT want an RO system. I already had an RO system and it was confusing with too much tubing, messy and difficult to change the filters, and took up way too much space. I also did not want anything that required electricity so I decided to go with the 3 stage first and then add this after it for under my kitchen sink.These filters are EASY! twist in, twist out. They are affordable!!For reference, I'm 50yo F and this was EASY to connect!! All of the Waterdrop's I purchased are EASY. an FYI, if you purchase this unit, any of the black single stage replacement filters will fit inside this. You can upgrade at your next filter change to a UF filter (ultra filtration) or downgrade to the 8k gallon filter for a yearly changeout instead. When installing this make sure you leave room under the filter if you want to upgrade it at any point. The upgraded filters are a bit longer.