Pond water acts like a big peice of sticky tape. Anything that falls from a tree, or is laying on the ground will eventually make its way into the water. Twigs, leaves, blossoms, seeds, walnuts, plastic bags, the neigbors garbage. I've even found barmitzva wrapping paper in my pond, which is strange for a small town in Iowa.
A pond skimmer operates in the same manner as a skimmer in a swimming pool. As water is drawn from the skimmer by a pump, water from the ponds surface flows into the skimmer. Debris is collected in a bag or basket of some sort. This is the first defense from keeping this debris from settling to the bottom of your pond and poluting your water. A skimmer should be installed in most medium to large ponds or in areas which have alot of trees or other ground clutter. The skimmer should be placed in the opposite corner of the pond from which the wind normally blows. In an established pond you will no what corner im talking about because the wind always blows the junk there.
The only drawbacks i've encountered with a skimmer is that I can't to grow floating plants on my main pond. I have also had small fish get caught in the skimmer and die (specially during spawning season). If you have large trees near your pond, the skimmer may not be effective during heavy leaf season, as an abundant amount of leaves can clogs the mouth of your skimmer, which will reduce the flow of water and could harm your pump. A skimmer doesn't make a pond maintenance free, and Sometimes it seems like more of a hassel than a benefit, but for those few days when it give you problems, it makes up for it during the rest of the year.
The following skimmer design requires that the water level in the skimmer must be below the top of the leaf baskets. Otherwise leaves will just flow over the baskets. This design will work with a submersable pump or an external one. If you use an submersable pump, use only one leaf basket so that you have room for the pump behind it. It is also possible to connect a bottom drain from the pond to your skimmer and put your submersable pump in your skimmer box. To get ideas what im talking about check out the commerically sold skimmer called PondSweep™ that sells for around $350.
