Ok, I put together the 250 mL. distilled water (thought I should try to do things right) and 14 1/2 tsp. Potassium Sulfate in a measuring cup. I made the dose 15 ppm - came out to 7 teaspoons and I only had a half-teaspoon measuring spoon.
I put 2.5 ml. in the tank tonight. The fish didn't like it so much. I'm a little surprised because there is still some K2SO4 not dissolved. I put it in the refrigerator.
Just a note; you may want to fix your code for your post.
As for your observation, you seem to be trying to dissolve a lot of solute into a small amount of water; that may explain why you are not achieving full solubilization.
You can try using more water (or less solute), and simply dose more volume (i.e. 5 mL or 10 mL instead of 2.5 mL) to achieve the same final result.