Posted: Wed Aug 10, 2016 1:51 am Re: Tank Care Questions
If the film is clear/oily looking, it's probably protein. To get that into the filter, you'll need something to stir up the water--air stones work, but power heads create a stronger flow. I think you can also buy protein skimmers. Another quick thing you can do is position a cup so that it's almost entirely in the water but empty, and then tilt the lip just below the water surface so the top layer of oily water slowly pours in. (This is more important in fish tanks, since that protein layer can inhibit oxygen exchange in the water, but I don't think it's exactly harmful to turtles--just a potential source of nitrates in the water, maybe).
I personally change about 30% of my water once a week (or more if I'm late on a water change). It sounds like you've got a good filter, although cleaning it once a month could help (just rinsing the sponges in a bucket of tank water, and cleaning out tubes). Even though I do all that, though, the protein layer still shows up (and I've got my turtle on a balanced diet, so he's not just getting protein-rich food every day). Good luck!