Matrix
What is it?
A filter media product sold by Seachem for providing a super-high surface area for bacteria to grow upon. It looks like fluffy grey/black stones of various sizes of about 1-1.5 cm in diameter.
Why?
Most filters use a form of sponge or ceramic discs or glass balls to provide a surface for bacteria to cling onto. But Matrix produces a modern far larger surface area than these traditional objects.
So in effect you can either reduce the amount of media in your filter to support a faster flow rate through your filter, increase your filter's efficiency without having to buy a bigger filter or by slowing the flow rate you can encourage the development of denitrifying bacteria which consume nitrate. Though Seachem do provide an alternative product called 'de-nitrate' which is very similar.