As a longtime aquarium enthusiast, I can testify to the amazing performance of these POLY SHEETS. If you have a sudden nitrite or nitrate spike, or any new and unwanted element discovered in your tank, insert a polysheet into your filter and problem solved! I don’t know how they work exactly, but they quickly remove unwanted water contaminants without destroying your good bacteria and not affecting your PH. I always keep the 12x12 one on hand as I can quickly cut it to fit perfectly into any of my filters. Leaves your tank water sparkling clear.