This may help.
if you shared 2-1/4 grams among all your tanks, then that would raise the potassium by 2 ppm.
There are 22 doses of 'water' in 250 ml when you dose @ 5ml/30 gallons.
I am coming up with 50 grams of K2SO4 mixed with 250 ml of water.
Dose @ 5ml/30 gallons to raise the potassium level by 2 ppm.