If you are keeping soft water fish....
Your GH and KH are best at not much higher than about 5 German degrees of hardness. Some of these fish appreciate the organic acids that peat moss adds to the water.
If you are keeping hard water fish....
The GH and KH are best well over 10 degrees, and many of these fish are best when they are in water that tests about 20 degrees each GH and KH.
Research the fish you want to keep and set the GH and KH to suit their needs. Most aquarium plants are fine in either sorts of water.
To raise the GH, add Seachem Equilibrium. This adds calcium and magnesium in a good ratio. It also adds potassium. Maybe not enough to count as plant fertilizer if you only need to add a low dose of Equilibrium.
To raise the KH add baking soda. Carbonates are a buffer that will raise the pH.
To lower these values blend tap water with RO water. It used to be said that you needed a minimum KH of 3 German degrees of hardness to keep the pH stable. This is a good target number. The nitrifying bacteria use the carbon from carbonates, and if the level were much lower the tank could crash.
On the other hand a lot of people have stable tanks with KH of about 1 degree. Quite a few of my tanks have a substrate that removes the KH from the water. But I am only doing DIY CO2. Not enough to really alter the pH all that much.
Fish do not care about the pH nearly as much as people used to think.
The daily pH cycle created by the plants does not bother the fish at all.
Changing mineral levels does bother the fish, and can kill them. When you have set up the tank the way you want, prepare the water for water changes ahead of time so the mineral levels match.