What are the ingredients of this "PMDD" recipe? The "silt-like stuff" is probably impurities from the K2SO4.
If you're worried about dissolving everything or dealing with aqueous solutions, then you should consider dosing dry. It's a bit less accurate but plants/algae aren't that sensitive IME.
If you're worried about dissolving everything or dealing with aqueous solutions, then you should consider dosing dry. It's a bit less accurate but plants/algae aren't that sensitive IME.