Kent Marine Nitrate Sponge is a granular zeolitic medium which works by two methods. 1) It slowly directly adsorbs small amounts of ammonia that are released by fish and natural processes, and 2) it fosters anaerobic bacterial denitrification due to its physical porous structure. This denitrification slowly reduces nitrate levels in the aquarium. It works well in freshwater tanks or tap water to remove toxic ammonia directly! Nitrate Sponge prolongs the time allowed between water changes by keeping nitrates low! 1 gallon bucket.