GH and KH depend on the livestock.
I try to keep them within about 2 degrees of each other, so...
Soft water fish that are wild caught or you are breeding: <3 degrees.
Soft water fish that are hatchery raised: 3-5 degrees.
Less picky fish: 5-9 degrees.
Hard water fish >9 degrees.
But it is best to research the species you are keeping to be sure it is in the right range. Fish go by GH. Set that right, then make the KH about equal, unless it is specified for the fish that they need something else.