KH and GH can be measured in parts per million, or German degrees (dH). The difference is dividing or multiplying by 17.9. A KH of 75ppm is the same as 4.2 dKH. 5 dGH is the same as 89.5 ppm GH.
If you want to know what hardness a fish requires, check it's recommended dH (GH/ dGH) at a reputable profile site like fishbase, and set your GH to match. My opinion about KH is to set it low, if you can, for the plants, and let the fish adapt to it. They depend more on a certain GH than KH. That said, many hobby species will adapt to whatever GH you have as well, stability and controlling fluctuation is more important than matching numbers from a site where some guy logged params from a certain collection site, those numbers should not always be considered 'requirements' for the fish. Many have a natural ability to adapt to changing params in the wild, and sometimes require that ability to breed and stay nomadic.
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