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Aquarium owners may at some stage notice a film that forms on the surface of the tank, looking like an oil slick. This is in fact a film of protein molecules formed from the breakdown of the fishes' waste. If not removed, it can cut down on light penetration to the tank bottom, especially in deep tanks, and reduce the efficiency of the gas exchange that happens at the surface of the water. This gas exchange is essential for the health of the aquarium's fishes, the water absorbing oxygen carbon dioxide being removed to the atmosphere. I noticed this effect in my heavily stocked cichlid tanks. A search online gave the advice of increasing aeration at the surface to disperse the film. I did this, to no avail. Other users advised drawing a piece of newspaper across the surface to remove the tank. Another suggestion was to remove the film by placing a glass into the tank and just submerging it so the film was sucked into the glass as it filled. Both of these manual methods worked, but were very time consuming and quite messy. I then came across the Eheim Surface Skimmer. This looked just the ticket. The surface skimmer (not to be confused with a protein skimmer: completely different piece of kit designed for saltwater aquaria) cost £23 from aquatics-online.co.uk. I was initially disappointed as what I appeared to get for my money was several pieces of cheap looking plastic and little else! Installation was straightforward. The skimmer is placed in the aquarium attached to the inlet of an external filter, held on by its own sucker pads. The skimmer connections are 12mm so connect directly to a 12mm hose from the filter. Larger 16mm hoses must use a T-piece (not included) to make the connection. The unit is placed so that the water inlet tubes form an angle of 45 degrees with the surface of the water. The skimmer operates by sucking water through the inlet, drawing in the protein film with the water, but without allowing air into the filter which can cause problems. Fluctuating water levels (evaporation/top ups) are not a problem as the unit can deal with a change in water height of around 2 inches. When feeding fishes with floating food, the internal float mechanism should be operated to cut off the surface flow avoiding drawing food into the filter. If the surface flow is cut off, either by operating the internal float, or if the mechanism gets blocked, water is drawn through the bottom of the skimmer unit instead, ensuring that the filter is not starved of water. I was pleased to find that within minutes of installing the skimmer, the thick oily film that had developed on the surface of my tank had been removed! With the skimmer in operation, the protein film has never been an issue since. The skimmer is quite large, and a bit unsightly, taking up a significant amount of space in the aquarium, but in my opinion, this is a small price to pay for ensuring that the horrible oil slick cannot build up. Other users have reported noise problems with the skimmer. Despite me using a powerful Eheim external filter connected to the unit, I've not had any issues with noise at all. The Eheim Surface Skimmer is not a cheap piece of equipment. For what you get, it seems quite expensive. It does work well, however, and can be recommended if you want to get rid of that horrid oily slick on the surface of your tank.