John, am I correct to assume the light is a single T8 24-inch tube, 6500K? The Life-Glo, or UltraSum, or similar? And what photoperiod? This data I need to assess the nutrient balance, as I'll explain below.
While waiting for that, to the surface protein film. Over the years I have had this, in a couple tanks quite heavy for a time, in others never. I relate it to organics, same as cyanobacteria, and in my case if I have cyano I always have the protein too; but I do see the protein, lightly, without any cyano. But both are related to organics, according to my research. Fish foods esp if overfed can also cause it [not in your case
] as can infrequent water changes; the latter makes sense as not doing water changes will naturally allow organics to build.
I used to use surface skimmers that attached to my canister filter intake (Eheim canisters, they make these attachments) on my three largest tanks, but took them off because the plants kept getting pulled in, and I lost several small fish and some fry. Screening the skimmer didn't help because them the plants clog the opening and the skimmer doesn't work at all.
I never bother with the protein film now; if I see any at the water change, I invert the Python and draw it off. But this is rare.
The fact that you only see it in a tank with very few fish [fish and fish foods being the most prevalent source of organics] might suggest too much fertilizer is going in the tank. I would cut back to once weekly with Flourish Comp and after say 3-4 weeks see if that makes a difference. This might also explain the algae in this tank.
Each aquarium is biologically different, even if we think we have uniformity in fish, plants, foods, light, etc. I had to reduce Flourish from twice to once weekly in only my 90g to prevent brush algae on the plant leaves, yet this never occurs in other tanks.
Edit: In your second PM you mention the lack of snails, in a Puffer tank to be expected, and this might be part of it. Interesting, now that you've raised it, the only tanks I've had this is my tank with loaches, so fewer snails. Snails do "feed" from the undersurface of the water, I see my bladder snails doing this all the time.