I have a 40gallon fish tank with sweet water, i got it almost a year ago and i have gotten alot of fish since then, in the beggining they kept dying because there were too many, now i have around 10 fish, big and small. They dont have a problem, and none of them are dying anymore.. but all of a sudden a wierd type of fog is appearing on the top layer of the tank, i noticed it when the light was on and i opened it, its like long wavey lines that move around, even though i cleaned the tank, they appeared after 5 minutes again, could it be bacteria? and how do i get rid of it