Aquarium Undergravel Filters
Aquarium undergravel filters never need cartridge changes and never wear out; they are tried and true, cheap, air-driven biological filters that provide some mechanical filtration as well. They filter your water through your gravel, so vacuum regularly to prevent nitrate buildup. Aquarium undergravel filters are best for small setups, or as supplemental biological filters in heavily stocked tanks. Pick up some lift tubes (below) to keep everything flowing, and a gravel vacuum.
Cartridges for some undergravel filters are available for optional chemical filtration.