Drop nitrates and phosphates as close to zero as possible.
This is different to what I have found by researching my infestation. My nitrates are 0, and I have loads of the stuff, I was told it can be caused by lack of nitrates (especially in planted tanks) so increasing nitrates by dosing KNO3 can work. I might be totally insane, but it's just what I've read. (And mine has improved from covering everything to covering half the tank since dosing) What are your readings for nitrate?