Re: potassium sulfate ?
Potassium sulfate (K2SO4) Is just a source for potassium (K). Since you are doseing Potassium nitrate (KNO3) you are already getting a source of K in your tank, depending on how much KNO3 you are adding you may or may not already have enough K. But my guess is since this tank is CO2 injected you are probably adding a fair amount and are getting enough K from your KNO3.
Also Just thought I would toss this in, spot algae, if its green spot algae mostly on the glass is usually a sign of either low CO2 levels, or low phosphate levels. You may want to make sure you have as much CO2 as you think in the water, and consider adding just a little more phosphate, say another 1-2 ppm per week.
As noted about though be careful if your phoshate is low and you add more, make sure you have enough nitrate in the water colum because relieving P stress will drive uptake of nitrate, and you dont want to bottom out your nitrates, it may cause plant stunting, and algae growth that may fool you into thinking that it was caused by the addition of the phosphate.
Good luck!
~Matt