Understanding
GH and KH
Two numbers that measure completely different things.
GH and KH show up on the same test kit, get read at the same time, and sound like variations of the same measurement. They aren't. Each one answers a different question about the water, and correcting one rarely fixes a problem caused by the other.
This guide explains what each measurement actually captures, why they can move independently of each other, and how to read both together instead of treating them as one number.
Two letters,
two different questions.
GH and KH sit next to each other on most liquid test kits, and get tested in the same few minutes. They share enough of their name that it's easy to assume they're measuring the same thing in slightly different ways. They aren't. They can move in completely different directions from each other, in the same water, at the same time.
This is usually where the confusion starts. A keeper adds a remineraliser to fix one problem, and a completely different parameter shifts instead of the one they were targeting. Or KH gets corrected and GH stays exactly where it was, because nothing about raising one necessarily touches the other.
The confusion isn't a knowledge gap so much as a naming problem. "Hardness" sounds like it should be one thing, and the test kit's two-letter abbreviations don't make it obvious that they're answering entirely separate questions about the same glass of water.
GH asks "how much dissolved mineral is in this water?" KH asks "how much acid can this water absorb before its pH actually moves?" Neither answer tells you the other.
Minerals for the fish.
Buffer for the pH.
GH (general hardness) measures dissolved calcium and magnesium — minerals that matter directly to living things in the tank. Fish use them for osmoregulation, shrimp and snails use calcium to build and maintain their shells, and plants draw on both as nutrients. GH is a biological number: it describes what's available for bodies and shells to use.
KH (carbonate hardness) measures the water's buffering capacity — how much acid it can absorb before pH actually shifts. It's a chemical number, not a biological one: nothing in the tank consumes KH the way a shrimp consumes calcium. KH's job is protective, holding pH steady against the acid that nitrification and other processes constantly produce.
Because they measure different things, they move independently. A remineraliser built for shrimp might raise GH significantly while barely touching KH. RO water starts at close to zero for both. Tap water in different regions can carry high GH with low KH, low GH with high KH, or any other combination — there's no rule that ties them together.
"GH is what's dissolved in the water for the tank's inhabitants to use. KH is what's dissolved in the water to keep the water itself stable. One is about biology. The other is about chemistry holding still."
Test both.
Match both to what you keep.
What counts as "right" for either number depends entirely on what's living in the tank. The first useful step is knowing what your species actually need, rather than aiming for a generic target. Soft-water fish and many Caridina shrimp species are adapted to low GH and struggle in anything approaching the mineral-rich water that hard-water species expect. Rift lake cichlids and most livebearers expect the opposite — hard, mineral-rich water they're specifically adapted to.
Signs of a GH mismatch tend to show up in the inhabitants directly. Shrimp show poor or failed moults, plants show nutrient-lockout symptoms despite dosing correctly, or fish never quite settle despite clean water and stable parameters. Signs of a KH problem tend to show up as instability instead. That's a pH that won't hold steady, or the kind of sudden crash covered in this site's guide on pH stability.
Test your source water's GH and KH before assuming a problem originates in the tank. A surprising number of stability issues trace back to tap water that's simply low in one or both, rather than anything happening inside the aquarium itself.
High GH with low KH, low GH with high KH, both high, both low — all four combinations occur in real tap water. Testing only one and assuming it tells you about the other misses exactly the cases that cause the most confusion.
Adjust each one
on its own terms.
If GH needs raising for the species being kept, a mineral remineraliser designed for that purpose does the job without necessarily touching KH. Check the product's own numbers rather than assuming it handles both — formulations genuinely differ in how much of each they add.
If KH needs raising to stabilise pH, crushed coral, aragonite, or a dedicated buffer product targets that specifically. Reach for this guide's companion piece on pH stability for the fuller picture of why KH runs out and how to rebuild it gradually.
The aligned habit going forward is testing both numbers together as a matter of routine, not just when something's already gone wrong. Treating them as one combined reading is exactly the assumption that leads to fixing the wrong one.
"Two numbers on the same test kit doesn't mean one story. Reading them separately is what actually explains what the water is doing."