I can't comment on the film on top of your water. Sorry. But, about the ph --- if your pool wants to stay at 7.8, I wouldn't fight it. 7.8 is within the acceptable range of 7.2-7.8. That will be just fine. If it goes to 8.0 or higher, however, you will need to add acid. But, if it likes to live at 7.8, so be it.
By the way, your pool is beautiful!
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