Well it killed all but one....it was during a critical stage of development so its not going to hurt adults but month old fry, definately. Ever since then I use plastic tubs to keep dust and stuff out and make the air humid.
I think it's mainly dust. I've tried everything but now I just clap the lids on or use plastic wrap.
The film is a product of micro organisms, it's pretty much protein, and it builds up in still water. The best way to get rid of it is to disturb the surface. Use a small bubbler or a filter, or do water changes to get rid of it. If those don't work, you'll have to resort to getting a cup and skimming it out. I've heard that apple snails will sometimes climb up and suck the water up top to eat that stuff, (though probably not enough) but if you have apple snails in other tanks, you might want to put one in there if you have the room. If you want to go uber fancy high tech, technically you could buy a water skimmer and that'd do the job, but those things are expensive
actually, I'm pretty sure you don't need an outlet for bubblers, though I could be wrong (I've never had one) . I always buy or find those outlet strips with at least three outlets and plug that in so my lights, filter, and heater can go there and anything extra can go on the other original outlet. (if that makes any sense)