When you do need to clean them it's best to do them in a bucket or cooler with plain pond water in it. Back flushing a filter with pond water is the best method but not all filters have that capability. I try and clean mine in the early spring and late fall, one good cleaning at those two times as in the spring you haven't been feeding through the winter and the fish aren't producing as much waste, same thing in the fall, you want to do this once the water has gotten to the point that you are not feeding anymore regular feedings, (Water temp below 55, I prefer to wait until water is below 50) Most of the bacteria aren't doing much then anyway and cleaning now will allow for good mechanical filtering while the bacteria aren't eating as much. One more thing, If your cleaning a filter during the active months it's best to do one layer and leave the rest, such as clean the foam but not the scrubbies, next time clean the scrubbies but not the foam. this allows for the rest of the filter material to keep a good load of bacteria in the filter and what you clean will regrow very quickly as there is still going to be a good amount attached to what your cleaning as long as you use pond water to clean them with, NEVER use the water hose that may contain chlorine or chloramine to clean filters, You will kill all the bacteria and that causes you to have to cycle the filter again, ALWAYS clean them with pond water! OH and when done put it in your favorite plants pot or near the roots, that stuff is super great for fert. house plants and garden plants!