I agree that the phosphates are the problem...I've been dealing with the same thing for the last six months. Finally making some progress...with a combo of more frequent water changes using phosphate-free water, reducing feedings, and running Phosban.
Do you have a DI (deionization) cartridge or are you running purely a RO filter? The DI will help tremendously to lower phosphates in your filtered water.
YMMV.
Chris