First thing you are confusing the (Red slime) for algae and trying to beat it by stripping the phosphates. It is actually a cyanobacteria that commonly will pop up if things are off balance. As Dowtish said try dosing a supplemental bacteria to counter it out. More than likely there is a bacterial imbalance causing the cyano and could also be causing your stn. And tumbling a large amount of gfo can cause stn as well.