Original or compatible?
A cartridge fits by its length, diameter and centre core, so it fits by the filter housing's model. Genuine elements come from the housing's maker; compatible elements (Unicel, Aussie Gold and others) are made to each housing's size and name the original code. Check the old cartridge's length and diameter as well as the model: some housings were sold in more than one size.
Where the model number is
The model is on the label on the filter tank or its lid (Zodiac CS150, Hayward C150S, Waterco CC100). On an above-ground pump it is on the pump's label or the box (Intex model 604 or item 28604, Bestway 58383); on an inflatable spa, on the box or the label on the pump unit (Lay-Z-Spa Miami, Intex SB-H20). If the label has worn off, measure the old cartridge's length and diameter.
Why change it on time
Hose the cartridge clean when the pressure gauge rises above its clean reading, as the filter's manual says. Replace it when a clean no longer brings the pressure down, or when the pleats are torn or flattened. The small cartridges in above-ground pumps and inflatable spas are replaced far more often: weekly to monthly, as Intex and Bestway set for each cartridge.
How this list is built
Every fit comes from the manufacturer's own compatibility list or parts page, with the source recorded on each row. Where only a seller lists a fit, the page says so. Prices are re-read from each seller's page daily where they publish one, and dated. How FitMatch checks fitment.