Inground and above ground pools require pool filters, in conjunction with pool pumps, to keep the water clean. The three most common pool filters are sand filters, cartridge filters and diatomaceous earth filters. Sand filters use packed sand in a pressurized canister to catch debris, and are the cheapest filter option for between $180 and $450 online. Cartridge filters clean pool water by trapping dirt in a cartridge, and cost between $200 and $800. Diatomaceous earth filters, also referred to as DE filters, use powder-coated grids that are able to trap the smallest dirt particles. DE filters cost the most at $280 to $800, but are the most cost effective pool filter.