I use one on my overflow and another on my skimmer output. Since I'm lazy I bought 6 and swap them out every 4 days, when the last set is dirty they all go into the washing machine with just bleach. I check each one as they're swapped just to make sure I'm not killing too much. Did find that hitchiker crab last week that I've been hunting for the last month...guess he decided to go "over the wall" on his own.
