There is no ideal KH or GH. With GH you need to be supplying enough calcium and magnesium for the plants. Even "liquid rock" water has been used with good results for both fish and plants. KH probably shouldn't be below 1, but I'm not even sure that is significant. Very high KH isn't good for the fish, from what I have read. Generally you can do more harm adding baking soda to raise the KH than if you just leave it low, especially since most people dump the whole baking soda dosage in at once. And, if you have the fish in the tank, after a water change, then add bicarbonate of soda to raise the KH, you are messing with their "osmotic balance". Incidentally, pH is also of little concern with either plants or fish.