Not quite... I am hoping that the concentrated solution dilutes in the surrounding water passively, without any help from spraybars.
Here is what I did last night, to get the testing started. I used two 3ml oral syringes, and one 0.5ml syringe. (With the markings on the syringe it is easier to keep track of changes).
I put 1/4 tsp of KNO3 in one and 1/4 tsp of K2SO4 into the other large syringe. That filled about 1.5 ml, then I added water which reduced the salt to 1 ml.
So now I have two syringes filled with 1 ml powder and 1 ml water. Similar for the smaller syringe which contains (less) KH2PO4 powder.
I placed all 3 syringes in my tank, with the opening pointing up. Now it is sitting back and monitoring how fast the fertilizer gets distributed into the water.
This morning (8 hours later) I did not notice any difference yet. The holes in the oral syringes are relatively large... perhaps this needs to be moved to an area of higher current if it doesn't dissolve fast enough.
I think this idea is mind-boggling in its simplicity. No ketchup or gallon bottles on top of the tank, just a (few) good looking glass vials with the powder and some water, small, easy to hide, still easy to check on...