What I was thinking was not to dissolve them (they won't, it's too much salt and not enough water), but to just let them settle down on the bottom of the vial. Therefore, you would have a near saturated solution in the container, which is just slowly diluted by tank water.
I imagine you could use bigger vials for KNO3 and K2SO4, and thinner ones for KH2PO4. Say you use 5 tsp of KNO3 for 2 weeks of dosing, you'd fill a thicker glass half of that, fill with water, let it settle, plug it shut and attach with rubber sucker in tank. Say you use 1/2 tsp of KH2PO4, you find a really thin vial, and do the same. Size of the diffusion hole would be the main regulation mechanism.