Why do you add twice as much K (in the K2SO4 dosing) as you do KNO3? I thought the general wisdom indicated equal quantities. Increased levels of K dosing might well have an influence on other uptakes causing, for instance, a shortage of something.
My water is softer than yours, but I did have a similar problem arise (thread and hair algae) when dosing Tropica Master Grow. After I started adding K2SO4 in equal quantities to KNO3, the issue was resolved. The only recurrence has been when the NO3 level dropped badly for one week.
I found that dosing K2SO4 seemed to increase the NO3 uptake so I increased the NO3 (and phosphate) dosing. So far so good.