Seachem Matrix Bio media is the most important part of our aquarium filtration system, so it is important that we choose the best bio media for the job. Bio media is where the vast majority of the bacteria that keep our aquariums clean and healthy live. This bacteria converts ammonia to nitrite, then another bacteria convert nitrite to less toxic nitrate. Nitrate isn't very toxic, but it does build up in the aquarium and can become dangerous to our fish in high quantities. So, to keep it low we change the water out in our aquariums to dilute it.