Finding a shimmering oily layer along the surface of your tank's water is a little alarming. While some hobbyists refer to this effect as an "oil slick," it is not the result of a petroleum spill. It's largely a cosmetic issue, but it can impact your tank's ecology over time.
The oily sheen represents an excessive buildup of organic compounds on the surface of the water. Most of these compounds contain both a carbon and hydrogen atom, although there are exceptions, according to the Reefkeeping Magazine website. Since these molecules are lighter than the rest of the liquid, they rise to the surface and form a distinct layer. Leftover fish food, dead organisms in the tank, excrement and decaying plant matter all contribute to the supply of organic compounds in the water. The oily sheen you see in your tank likely didn't happen overnight; it was probably the result of days or weeks of organic buildup. Cooking spray, olive oil and similar foreign compounds can rise in vapor over your tank, accumulating on the water's surface.
The so-called oil slick does not present an immediate or severe danger to your fish. However, the sheen represents an overabundance of organics, which isn't good for your tank in the long term. The organic layer disrupts the normal exchange of oxygen and other gases between the water's surface and surrounding air, causing a gradual shift in the water's chemical composition. Organic compounds can also catalyze the growth of bacteria and other undesirable aquarium residents, which can further disrupt chemical balance, according to the Aquariums Life website. Of course, display tanks lose some of their aesthetic appeal when the water looks oily and dirty.
Risk Factors and Prevention
Overcrowding is often cited as a source of oily layers in fish tanks. Consult an aquarium expert when you select your tank and its occupants to make sure the habitat's big enough for your planned ecosystem. Ask him about appropriate meal sizes for the residents according to their species; research otherwise to determine what and how long to feed -- different fish have different requirement, regardless of what your jar of flakes' label says. Leftover food is messy and a common source of excess organics in aquariums. Dead or dying organisms are also a significant source of free-floating organic compounds, so remove dead feeder fish if your aquarium's residents don't consume them after a day or two.
To alleviate the problem in the short term, change up to half of your tank's water over several days and use a skimmer along the surface regularly. Reduce feeding temporarily and check for decaying plants, fish and other tank residents. This might reduce the organic buildup temporarily, but you'll need to increase your tanks aeration as well as the level of surface disruption to stop the problem from returning, according to Aquatics Unlimited. Installing a protein skimmer and keeping your tank's filters clean also helps keep well-stocked aquariums clean.