I use the four fertilisers from gemgardening

1. sulphate of potash (K2O)
2. Nitrate of soda (NaNO3, sodium nitrate)
3. Superphosphate (P2O5, phosphorus pentoxide)
4. Garden Lime (CaO, Calcium Oxide). This when mixed with water becomes
Ca(HO)2 calcium hydroxide, which then combines with CO2 in the water to form
CaCO3 Calcium carbonate which increase my KH and GH.

Ideally I would like some K2SO4 and KNO3 and also micro nutrients without
any iron. But in the UK its impossable.