General Hardness (GH) and Carbonate Hardness (KH) don’t get the attention ammonia and pH do, but they quietly control how stable your tank actually is.
KH is your water’s pH buffer — low KH means pH can swing wildly with small changes in the tank (a bit of driftwood tannins, a missed water change), even if everything else looks fine. If your pH seems to drift for no obvious reason, check KH before you touch anything else.
GH affects fish health more directly, especially for species with specific mineral needs. Guppies and livebearers generally want harder water; soft-water species like many wild bettas struggle in the same conditions.
Test both once a month in an established tank, and any time you switch water sources (new tap supply, switching to RO water, adding a remineralizer). A GH/KH booster is a cheap, easy fix once you know your baseline — the mistake is not knowing the baseline at all.