Clear film over top of reservoir?

[Reefer08]

Im getting a clear/milky looking film on the top of the water in my reservoir. It breaks up just like a layer of thin ice. I mix kalkwasser in the reservoir so I believe its caused by the kalkwasser. Any why this occurs? Thanks

[HighlandReefer]

The top film is calcium carbonate which is precipitated out of kalk water due to its high pH. It is not a concern. ;)