From what I have read, the Potassium provided by KNO3 is not enough to reach the "optimum" 20 ppm level. Also, when you get a higher fish load, you will dose less NO3, but still need the same amount of K.
Having said that... If your plants look healthy and you don't have other (algae?) problems, I would hold off with K2SO4.
If you start to see holes developing in older leaves (in swords, for example) you might want to start looking for K2SO4.