With our fish friendly skimmer options you won't have to worry about finding one of your favorite fish trapped and injured behind a skimmer weir door or stuck in your skimmer net. The efficiency of any biological waterfall filter is determined in part by mechanical skimmer pre-filtration. There are several important characteristics to a good skimmer pre-filter that you will need to take into consideration. Fish friendly design, ability to disguise the skimmer into the landscape easily, simple installation and easy to maintain. A small piece of slate of flagstone can be used to cover the top of the skimmer easily.