I have two.. I put one in place when I do my weekly water change.. They'r great for catching the stuff you loosen up off the glass and overflows, but I've noticed that they quickly become clogged after no more than 24 hrs and the water flows out the top rather than through the sock, great for keeping the water clear, but I remove them shortly there after.. I've been told they suspend your waste thus raising your phosphate levels, thats another reason I don't leave them in longer than that.... Hope this helped...