K2SO4 is often used as a substitute for KNO3, since fish (and other critter) waste often adds enough NO3. That way you get the required K without adding more unneeded NO3. Yes, if you are dosing correctly, you will get plenty of K from the 2 ferts you are dosing.
If you have a high fish load you may want to consider using K2SO4 ... I actually use a combination of K2SO4 and KNO3.
