Thin white film on the inside of the glass - How to remove?

[Kmst80]

It is still likely to be calcium carbonate, which in an empty tank is easily removed with acid such as vinegar.

Otherwise, it is organic matter that is best removed, in an empty tank, with bleach.

In an operating tank, it’s much harder and the tools have been mentioned above (razor blade, scrubbers of various sorts, etc).

+1 on Razorblade , just gotta be real carefull not to scratch the glass. I get a bit of that while calcification on the watersurface and razorblade takes care of that.