Bio-film

What is it?
Bio-film (biofilm) is a thin film on the surface of aquarium water, caused by the build up of protein from organic waste material. It is the structure bacteria build to support themselves growing on the surface where they get access to oxygen and the material.

What does it look like?
It can be spotted by looking at the surface at an angle to a light source and you may see reflected an oil like rainbow effect. It can also be directly seen by breaking up the surface of the water with a finger or stick to make bubbles. If bubbles form and last for more than a few seconds then you've got a bio-film.

Is it dangerous?
In certain circumstances (high bioload) it can reduce the level of oxygen to the rest of the tank. It can also stop CO2 getting released from the water so CO2 levels rise and so may cause asphyxiation to the animals in the tank including the nitrifying bacteria in a filter.

Causes

 * It can be caused in a new aquarium where the gravel release their organic debris and it settles on the surface.
 * It can be caused by a male Betta, Paradise or Gourami fish when he constructs his bubble nest. He uses his own mucus to form the bubbles.
 * Tiny uneaten food particles or decaying plant may rise and gather at the surface.
 * Add too much bottled bacteria may cause this.
 * Unfiltered tanks or tanks with very little surface movement are more prone to this than well filtered tanks.

Removing it
Ensure an adequate water flow at the surface or provide aeration to break up the water surface. It will soon dissipate. In the immediate instance a fishnet can be used to take the film off the surface of the water.