I actually just had this issue on a tank that is very well kept up. I bought a magnum 350 like others have suggested above. I ran the cartridge overnight, cleaned it and put it away for future use and just use the magnum as a regular canister now. Water was spotless the next day and it's been about a month and it still looks great. I have not been able to figure out what the issue was. I have six other tanks I take care of in the same manner and no problems with them. It was just this one tank. I believe the magnum was worth the $ I haven't had to use it's cartridge again yet but won't hesitate to. It was a bit of a pain to put together though. But well worth it.