If your pool's pressure gauge is reading a higher PSI than normal and the jets seem to be less powerful, or you just have had it for a while, you may need to replace your filter. It is recommended you replace your filter every few years or so, dependent on use and how well you take care of it. This is a set of 4 filters, each being 105 Sq. Ft. in size. Sold as the set of 4 only.