There is an oily white thin film on the water surface (cycling tank) ???

There is an oily white thin film on the water surface (cycling tank) ???
what is that ?
is it harmful when i put fishes?
( skimmer is switched off during cycling )

 

Probably protein from decay and what you cycled with. I wouldn't worry yet. Get a powerhead pointed at the surface and get it stirred up. But ya don't want that when you get fish. The skimmer should help.

 

You don't want slack water on the surface at any time. Point a return or a PH up at it so it ripples and the tension is broke. This allows for proper gas exchange.

 

This stuff will appear in all tanks (freshwater or saltwater) with lack of surface agitaton. It's basically protien in the water settling on the surface. Surface skimmers work well to remove this in tanks that have little surface movement. Just point your powerhead up to the surface and get the water moving, it will then get broken up and mixed back into the water. There is much more of this protien during cycling so you probably have a whole bunch of it in your tank. It will become less and less as the cycle finishes but there is always a hint or more of this stuff in all tanks.