Stability is a good thing in an SPS tank, of course. What I'm getting at, though, is that there's a point at which more frequent additions doesn't make much difference. One could, for example, use a positive displacement piston pump to dose additives on a continuous basis. But that's not necessary, and the pump would be expensive and loud.
As Kris notes, there's no reason why you can't dilute your stock solutions if you choose; the only limitation to how far you could go with that is that you stay under your evaporation rate.