Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
In the wild water gets its Ca and Mg (we measure as GH) and its carbonates (we measure as KH) mostly from limestone and related rock. Limestone and related materials are calcium and magnesium carbonates and bicarbonates with minor additives, other minerals.
Most natural water has GH and KH sort of similar, though there are certainly variations!
Fish evolved to handle that sort of water.
If the GH is low, then the KH is almost always low, and the pH generally follows the KH, though there are other things that may control the pH.
Soft water fish generally thrive in water that is neutral to acidic pH.
If the GH is high, then the KH is almost always high, and the pH generally follows the KH, and is in the alkaline side of neutral.
Hard water fish generally thrive in water that is alkaline.
This is not a hard and fast rule, just a generality.
Look up the information about the natural waters your fish came from and see what are the optimum values, and the range of values. As long as the fish are kept in the right range of values they ought to do well.
What kind of fish are you keeping?