The Fluval Auto Surface Skimmer SK400 removes unwanted water surface film to help improve oxygenation and light penetration for a cleaner and healthier aquarium. Over time, surface films can develop from oils on your skin, household dust, cooking smoke, fat content within fish food and more. These films are not only visually unsightly, but also reduce dissolved oxygen (required by fish to breathe) and decrease light levels (needed by live plants to grow).