Seachem Matrix biomedia 2L. 4 per case. Is a high porosity biomedia that provides efficient biofiltration for the removal of nitrogenous waste. Each liter of Matrix provides as much surface (internal and external) as 40 liters of plastic balls! Plastic bio-materials provide only external surface area, whereas Matrix provides internal macroporous surface area. These macropores are ideally sized for the support of nitrifying and de-nitrifying bacteria. This allows Matrix, unlike other forms of bio-media, to remove nitrate along with ammonia and nitrite, simultaneously and in the same filter. As long as the water is well prefiltered, Matrix should not require replacement. Since the majority of the bacteria are internal, Matrix may be rinsed when needed without damaging the filter.