It somewhat depends upon plant species and perhaps the method. The GH is indicative of the level of "hard" minerals like calcium and magnesium primarily, and the faster-growing plants obviously need more of these. Calcium is a major nutrient in cell structure. When this mineral is lacking, the plants will take up iron in its place, causing an excess of iron showing as brown patches which increase until the leaf and eventually the plant is dead. I've had this issue with my very soft water, GH less than 1 dGH at around 7 ppm. Slow-growing plants have no issue as I add Flourish Comprehensive and this supplements the hard minerals sufficiently. But the larger swords and red tiger lotus, plus most of the stem plants, will not manage without increasing the GH to provide the hard minerals. A GH of 4 dGH is suggested by Walstad, and in my case I use Seachem's Equilibrium to raise the GH as this adds five minerals without affecting KH or pH. It took me several weeks to get the dose worked out to maintain a level between 3 and 5 dGH depending upon the tank and the plant species; the plants rebounded impressively. You don't want to over-do it, as there is no additional benefit, and the higher GH may affect certain soft water fish. I settled on the differing levels for each tank solely based upon the condition of the plants over several months.
Byron.