I found it!! It's been driving me bonkers plus I was worried it was hurting my little guy. I've had him since last summer and never had a single problem, until this mystery. For weeks, this weird film scum would start to develop just days after the water change and it covered the surface of the water. I'd literally taken to dragging a paper towel over it to mop it up - I didn't know what else to do.
Then I thought, the containers I use to keep the water in prior to changing the tank water had possibly becomecontaminated. They were the only thing that I hadn't cleaned regularly. So I gave the containers a thorough washing & rinsing and
... so far so good. It's been a few days now and the film hasn't started to re-appeared at all. YAY!!!
Hopefully this is a fix. Will follow up with an update... fingers crossed!