What is scum skin?
In some aquariums, a layer that resembles an oil film forms on the water surface. This so-called scum skin can take on a dull green color, but does not shimmer in the colors of the rainbow. If the scum is skimmed off, it usually forms again after a few days.
Some species of fish such as black tetras, guppies, platies and angelfish occasionally eat scum or the bacteria living there by sucking in parts of the layer. This usually only happens if the fish have not been fed for a few days.
The scum can become so thick that it forms blisters that dry up and harden if not removed.
The scum and eating the scum is not harmful. However, the gas exchange between air and water can be impeded. This can even be an advantage if it means that less CO2 needs to be supplied to feed the plants and maintain a certain pH value.
Caution may be required with unregulated CO2 systems. It must be ensured that the CO2 content of the water does not become too high due to the lower gas exchange.
What is the scum in the aquarium made of?
The scum is a biofilm consisting of a mixture of organic matter and bacteria. In open aquariums there is also dust that falls to the water surface due to the lack of cover. Fish food and faeces contain organic substances that tend to accumulate on surfaces, e.g. B. at the water surface. In addition, there is ammonium and nitrite dissolved in the water.
The organic substances, as well as ammonium and nitrite, are eaten by bacteria, which therefore also settle on the surface. At the boundary between water and air, both a mass exchange with the water and a gas exchange with the air are possible. The surface thus offers an ecological advantage for various types of bacteria. These are the same bacteria that mineralize leftover feed and other organic matter in the filter. Dust also consists to a large extent of organic substances that are eaten by the bacteria.
The thickness of the scum thus depends on the amount of organic substances in the aquarium water. Since the amount of organic matter is essentially dependent on the number of fish and the feeding, scum can indicate that the fish stock is too large or that too much is being fed. In this case, the bacteria in the filter and on the surfaces of the aquarium are not efficient enough to break down the organic matter.
A scum skin may also form if the animal is fed frequently with live food. The high protein content, e.g. B. responsible for Gammarus. Over-fertilization or the death of large fish can also cause the water to become overly polluted with organic matter. Heavily fertilized and poorly filtered plant tanks may also be prone to mold growth. Apparently the scum is also temperature dependent. When the temperatures are high in summer, a scum skin can form, which disappears again when the temperatures drop.
Remove scum with a skimmer:
How do you prevent scum?
The deposition of organic substances present in the water on the surface cannot be prevented. The formation of a scum film can therefore only be prevented if there are as few organic substances in the water as possible.
With the following measures, the amount of organic matter can be reduced or existing scum can be reduced:
- Reduction of the fish stock
- reduce feeding
- Fertilize less
- Increasing the stock of plants with fast-growing plants
- Cover to prevent dust from falling to the water surface.
- Using a surface skimmer, suck off the top layer of water and direct the organic matter into the filter.
- Use bubble snails. These eat the scum skin.
- Absorbent paper, e.g. B. Place newspaper or kitchen towels on the water surface. The paper soaks up the scum.
Increasing the number of plants or using a surface skimmer reduces the CO2 content in the aquarium. Under certain circumstances, CO2 fertilization must be used to ensure that the plants have enough CO2 available. Another disadvantage of surface skimmer is that they do not work silently.
With the often recommended measures of increasing the flow rate of the surface water or moving the surface with an oxygen stone, the scum is not removed, but only pushed together by the current. In addition, CO2 may be expelled from the water if the current is too strong.
The second scum skin
Also known as scum is another layer on the water surface that shimmers like an oil film and consists of iron bacteria, various species of which live in iron-rich soil and water.