Some fish and some plants will do better in hard/soft water, but generally they all do fine in a wide range of both GH and KH. Your local fish store probably keeps their fish in tap water, so you know those fish are acclimated to your tap water.
One possible problem with GH is that if yours is from calcium ions only, with little or no magnesium, you do need to dose magnesium. Most of us, I think, use a GH builder, which contains both calcium and magnesium (and potassium, usually) at water changes to be sure we have adequate magnesium.
Low KH is very good for many plants, and doesn't harm the fish. But, high KH is tolerated by fish when they are acclimated to it. Low KH would be 1-3 dKH or so, and high would be above 6-8 or so. I use water that mostly comes from the local river, and it is soft, low in both GH and KH. I don't bother to measure either one, and I ignore KH entirely.