PennPlax AmmoniaX Undergravel Filter Cartridge (2 Filters Per Pack) Ammonia Removing Under gravel Filter Cartridges are made with top quality zeolites which work to eliminate the top fish killer ammonia Under gravel Filter Cartridges solve the problem of overfeeding, over-crowding and foul water conditions Filter Cartridges fit all Under gravel filters Cubes: 0.0007. Weight: 0.2500 lbs