gf225 wrote:

Does anyone know where in the UK I can buy Potassium sulphate (K2SO4)?
If not does anyone know where I can buy Muriate of Potash (KCl)? If so
does anyone know how soluble either of these are? I'm after dosing
potassium as well as nitrates and phosphates.

Try your local supermarket/grocery. If you're lucky, they'll sell
sodium-free water softener salt (check the label - it's probably KCl).
In my local store, it's only about twice as expensive as regular water
softener salt - i.e. more than you can ever use in a tank for pocket
change.

Solubility of KCl is very good - 300+ g/L
K2SO4 is ~ 120 g/L