filter socks are made of a special material, they really help with polishing your water and killing micro bubbles. They also need to be changed out very often or they can act as a nitrate sponge. I change mine every 3 days, turn them inside out and run 1 washing machine cycle with a 1/2 cup of bleach, and then 2 rinse cycles of just cold water, air dry and they are good to be changed out with the dirty ones.