The past two weeks or so, I've been noticing a weird film forming on the top of my fish tanks. Looking from above, the water looks cloudy, but if I move the water it goes into clumps that almost look like wet paper. Two of my fish don't seem to be bothered by it, but the third seems a little bit less active than usual. One of my fish had a problem with reoccurring SBD not too long before this, but I don't think it's related because he hasn't had any symptoms for several weeks.
I did a search on this forum, and it kind of sounds like protein film to me. Am I right about that? Is it something to worry about, or does it just mean I have to clean the tanks more often?