The Nitrate Sponge is a granular zeolitic medium, which works by two methods.It slowly directs and absorbs small amounts of ammonia that are released by fish and natural processesIt fosters anaerobic bacterial de-nitrification due to its physical porous structureThis de-nitrification slowly reduces nitrate levels in the aquarium. It works well in freshwater tanks or tap water to remove toxic ammonia directly. The Nitrate Sponge prolongs the time allowed between water changes by keeping nitrates low.