KH and GH are two totally separate measurements. KH measures carbonates, GH measures calcium and magnesium ions. So the numbers are completely independent of each other.
You can easily raise KH by adding baking soda (sodium BICARBONATE). Since this contains no calcium or magnesium it will have no effect on GH.
You can easily raise GH by adding epsom salts (MAGNESIUM sulfate) or CALCIUM chloride. This obviously has no effect on KH since no carbonates are involved.
Adding CALCIUM CARBONATE will raise both KH and GH.
GH is usually what planted tanks need, since plants need both calcium and magnesium. It's a matter of debate whether plants benefit from KH, many people (myself included) keep plants successfully with a KH of 0.
Thank you very much Carissa for that information. I was primarily concerned with KH because I understand pH can swing wildly without some buffering. I haven't added fish yet but foresee doing that in about a week. But I'm less concerned with that now. What is a "optimal" GH for most tropical aquatic plants?
Whether pH swings has more to do with whether you are doing good maintenance than anything else. If you are doing weekly water changes of 30 - 40%, it's unlikely that your pH will swing unless you have a very high fish load. However if you tend to go for a month without water changes, yes in that amount of time the pH can crash. I've had this happen, but never lost fish over it. Adding KH has it's risks too, because a slight mismeasuring from one water change to the next will cause a KH/pH swing. KH fluctuation is much more harmful to fish for the most part than pH fluctuation. So I don't know if I would jump on that bandwagon, if I were you. Once you're on, it's not easy to get off. However, on the other hand some fish require a higher KH to thrive, so in that case maybe you should if you are keeping those types of fish. Personally, I choose fish that suit my water type. Tetras and angelfish do great in soft water.
What you should do, though, is make sure to drip acclimate your new fish. This will prevent them from going into shock if they are coming from harder water to your softer water.
Thank you again. I agree and don't plan to mess with the KH, and my GH is already where is should be. I will choose fish that are best suited for these values. I will also be careful to use drip acclimation.