Freshwater skimmers havent really been perfected for aquarium use yet, i wont try to explain the mechanics of them as i dont know exactly how they work myself but it involves injecting huge ammounts of air at high pressure into the water to produce the tiny bubbles needed to skim proteins from the water. They have them for ponds but they are big noisy contraptions so you wouldnt really want one on an indoor tank. The process works in salt water because the higher specific gravity makes the water denser and the air bubbles smaller so less pressure is needed.
What skimmers actually do is removed organic waste from the water before it can break down and enter the nitrogen cycle, so you get lower nitrates and less dissolved organic solids in the water.
