I have the same problem and was told it is because of high Phosphates. I have been using the water polisher that came with my filter (H.O.T. Magnum) and that clears it out, but it comes right back. I was told to use a phosphate remover, so I got Phos-X. But i can't fit it into my media container. The Magnum has a D shaped container with a tube running through the center, so I can't fit the Phos-X bag around it.
Are Phosphates the problem? Is there a way to get the Phos-X into the container? Is there something else I could use?
I have a 55 gal, you can see what fish I have in it in my sig, my pH is about 6.8, 0 NO2, about 40 ppm NO3, 0 ammonia, Hardness 50 ppm, alkalinity 0. I don't know what the Phosphate level is I don't have a Phosphate test (the LFS did the test). I did a 20% water change yesterday and have the water polisher in right now.