Film on the water surface or 'oil slick' in the aquarium is a very common topic while browsing aquascaping forums. There are many different reasons for surface scum and a variety of methods by which to remove this oily film from the top of the water.
There can be two main problems on the aquarium surface: the first is a bacteria scum, the second is a thick layer of biofilm. Sometimes it happens that these two issues are related. In the case of an oily film or biofilm on the top of the water, it is important to pay attention to the time it remains on the water's surface. If it is short, a mistake in the maintenance (e.g. when changing the fish tank water) is to blame. The situation can be dealt with quickly or the problem will be resolved by itself. If the biofilm or oily layer persists for a long time, more comprehensive actions need to be taken.
Film during water change or cleaning
Biofilm on the water surface often appears during a water change, most often due to the formation of a large number of air bubbles. Oily film formations are somewhat natural when water is poured into the aquarium inadequately (e.g. from a great height, quickly and directly). In addition, the foam quickly "collects" impurities set in motion, resulting in an unaesthetic, threatening-looking sediment on the glass. It also happens that detergents for washing dishes, windows and floor is used to clean the aquarium glass or other elements (even buckets used for a water change). You have to remember that even a small amount of it introduced into water will create an oily film. Biofilm may also be present when starting the spraybar (at maximum efficiency). In any of these cases, it is harmless.
So how to avoid film during cleaning or water change?
Water should be poured into the tank carefully, slowly, e.g. along the glass, on a plate placed on the bottom or on thick leaves (e.g. anubias). If there is a small amount of detergent in it, it will be broken down by the filter. Of course, it's better to avoid such situations, and a lot depends on the type of detergent. Will I need a water change? Such situations cannot be completely ruled out. The spray bar can be installed slightly lower and the pump performance reduced to avoid generating excess bubbles.
Long-lasting film on the surface
The formation of a thick layer of biofilm or oily layer is often a sign that there is an excess of protein in the aquarium water. The particles stick together and collect at the surface water. This is a problem, especially in saltwater aquariums, so the basic equipment is a protein skimmer. In freshwater aquariums, protein films are less common. Today we see more aquascapers introduce surface skimmers to their planted aquariums. These are slightly different from a protein skimmer however, they can be effective in removing an oily film from the top of the water. Surface skimmers can also help gather up any trimmed plant leaves (especially thin, needle leaves)
An aquarium is an ecosystem and all causes are correlated. We have listed the most common causes of the film-forming.
More fish in the aquarium means extra food and as such, every feeding time means more fish poop. There is a common rule that 1" of fish per 1 gallon of water is accurate. Of course, everything depends on the species you choose. Especially when talking about territorial species.
Too much fish food means that they can't eat it within a few moments (2-3 minutes). The uneaten fish food drops on the bottom and is an additional organic matter, decomposing in the aquarium water. The protein film may appear more with certain types of fish food than others.
Too few plants
Plants are a natural filter for the fish tank, changing CO2 produced by fish breathing or CO2 fertilization into oxygen, which is the byproduct of photosynthesis. Moreover, plants absorb Nitrogen produced from fish poop, supporting a healthy fish tank filter at the same time.
Your fish tank Filtration system should be large and powerful enough to cope with all the fish in your aquarium. Always ensure your filter is powerful enough to circulate effectively by checking the manufacturer's guidelines. A biofilm or oily film is likely to appear on the aquarium surface if your filter is not maintained correctly by regular rinsing of the filter media. It is very important to remember to clean your filter media with aquarium water, not tap water or distilled water. You would rinse beneficial bacteria living on the filter media.
Directing the filter outlet in a way that promotes surface water movement can have a some extra benefits when dealing with bio film on the surface. Water movement plays a huge part in breaking down the film and will allow an effective gas exchange at the surface of the aquarium water.
Sometimes we experience a fish death. It does happen to us all but a decomposing dead fish is another starting point for oily films on the surface. Therefore if you notice a dead fish in your tank, take it out immediately.
How to fight protein film?
Primarily caused by over feeding with lower quality, oily food, to avoid such an oily film, a much larger water change than normal should be performed. It is also possible to manually remove protein film or an oil film by using paper towels. Due to the absorbing powers of paper towels, simply placing them directly onto the surface, the paper towels will lift oil films away from the water. Although this method should only be used in the short term. The most important thing, however, is prevention. To do this, we have to search for various sources and eliminate the cause.
It is important that the fish have the necessary space to live. A freshwater aquarium should be densely planted with greenery. If your aquarium plants are growing poorly, excessive fertilization is not the solution. Try to improve your fish tank lighting and turn it on regularly (even 8-10 hours a day) along with CO2 injection. If your fish are known to destroy the greenery, it is worth avoiding the delicate species and choosing the more robust ones that are easy to grow. The above-mentioned anubias work well in both cases. It is recommended to feed the fish several times a day, but in small doses. The fish food should not sink to the bottom (unless we want to feed bottom fish such as corydoras. It should disappear within 1-2 minutes. Filter media should be cleaned regularly, preferably with water straight from the aquarium (this way beneficial microorganisms, e.g. nitrifying bacteria, are not destroyed). Dead fish should be removed instantly.
To sum up, film on the surface may be caused by various sources. Firstly we need to identify them to choose the correct way of improving the water quality. Maintenance is key here. Following basic rules of keeping fish should help you to avoid the problem along with performing the regular water change in proper way described above.