There can be one of three types of the pellicle that forms on the water surface: oiliness, jelly-like (artificial origin), jelly-like (colonies of unicellular organisms or bacteria).
The first type of pellicle occurs quite often in new tanks. This is the result of insufficient drying of the putty that covers varnish, and the substance varnish contains goes out in the water.
Sometimes when the posts or the framework of the tank are covered with nitro dye and there is no aeration inside, the second type of pellucid may appear. In both cases it is better to change the water completely.
In unclosed tanks or owing to overfeeding the third kind of pellucid forms. It arises in the presence of poor circulation. In this case you should acquire a powerful water pump or an air pump that will provide the current of water and reduce the risk of this pellucid. Besides, the trouble can be done away by means of highlighting and regular hygiene measures (cleaning of the substrate, refreshing water and so on) that lower the concentration of the organic matter in the water.
Quite an easy way to remove the pellucid is as follows. Carefully put the sheet of paper on the water surface. The pellucid will stick to it. Slowly pull the paper over holding it by one of the sides. Repeat several times.
If none of the measures helped, check up the quality of the filler in the filter. A bad filtering material may dissolve in the water and also cause the forming of the pellucid.
Translated by Tatiana Karpova (Moscow)
(MSU, Biology faculture, Dep. zoology and ecology).