When we bought our pool 3 years ago we went with a cartridge filter. It works good but have found it to be a real pain when opening the pool and during the once a week of pulling it out and cleaning it all the time and the $100 for each cartridge which after opening I have to get another one and by the end of the year spent about $300 anyway so we are going with a sand filter but not sure the size we need. we have a 28' round pool 4' deep.