I've noticed that over the past few weeks my tanks have gotten this weird oily looking film at the top of the water. I usually just scoop it out, but I was just wondering what the cause is? Is it because of the filter cartridges in the filters getting dirty? Or is it because the water at the top of the tank is really still? Some people have said that it could be from the plants, but it had been happening since before the plants so I don't think thats the issue. If its because the water is still, what should I do, because I thought Bettas don't like water that is constantly moving.