I have one male Betta Fish that I keep in a 2.5 gallon tank with a heater, moss ball, and various other fake soft plants. I do 50% water changes each week. But I noticed that the very top of the tank develops a slimy film after a few days or so. My little guy seems just fine, he’s very active and eating his food like usual. I’ve read that it develops when the water is still? Is that it? Or is it something to be concerned about? I’m new to this and just want to be sure!