At face value, their advice is inaccurate and misleading. It depends upon the species in some cases, and obviously the actual parameters, but fish in a store tank have not adapted to that water. It takes weeks, if not months, for any fish to adapt, and some cannot regardless. The fish in the store are there for limited time, and most species can manage for this brief period, again depending upon the species, where it comes from, and the actual GH/pH of the water involved. When you get the fish home, you are intending to keep it for its life, and that means providing the water parameters it needs. Otherwise the fish will be stressed, it will weaken, it will be more susceptible to disease, and it will have a shorter lifespan. Studies done with the cardinal tetra in the late 1980's in Germany confirmed that the lifespan of the fish was in direct relation to the GH of the water; the harder the water, the shorter the lifespan. And when dead, necropsies found that the fish died from calcium blockage of the kidneys. The high calcium in the water gets inside the fish as it "drinks" by water passing through the cells, and the kidneys attempt to filter out the toxic calcium, and get blocked.