Most plants like soft water, but a few do not. I usually try to raise the GH by a degree or two with GH booster. Then the water is still in the soft range, but should also have enough GH to avoid deficiencies. The best thing you can use to raise GH is GH booster, because its premixed to contain more or less the correct proportions of Calcium, Magnesium, and Potassium which are the main constituents of water hardness (GH).
Another thing to keep in mind is since we have very soft water you shouldnt add K2SO4, becasue the the K needs to be in (a not very strict) balance with Ca and Mg in order to avoid Ca/Mg deficiencies. This is especially true with high light/co2/ferts. You should be getting more than enough K from KNO3 and KH2PO4 if you are dosing EI.