I've noticed over the last 3 months or so that the surface of my water is quite nasty. I do a weekly water change of about 20%. When doing so, I use a good sized bowl to skim the surface, removing the vast majority of it. By the time the following week comes around the surface is, again, quite covered with a soupy mix of gunk. I have no idea what it is. I feed spectrum pellets and nori flakes, alternating the two daily. Is this normal? Am I missing something?