Aquarium GH and KH Explained
GH measures calcium and magnesium for fish health; KH is carbonate hardness that buffers pH. Learn ideal ranges and how to raise or lower each safely.
GH (general hardness) measures the calcium and magnesium your fish and plants need, while KH (carbonate hardness) measures the carbonates that buffer your pH and keep it stable. They sound similar, but they do different jobs, and testing both is one of the most overlooked habits that separates a stable tank from one that crashes.
Most beginners obsess over pH and never test hardness, then wonder why their pH swings or their shrimp will not molt. The answer almost always lives in GH and KH. Once you understand these two numbers, water chemistry stops feeling like a mystery.
Hardness Testing and Adjusters
$13.49 on Amazon
Liquid titration test reads both general and carbonate hardness drop by drop.
$28.99 on Amazon
Slow-release media that raises both GH and KH and buffers pH against crashes.
Brightwell Aquatics Remineralizer for RO Water
$15.58 on Amazon
Adds calcium and magnesium to raise GH when starting from RO or RODI water.
$23.99 on Amazon
Quick strips for routine GH, KH, pH, nitrite, and nitrate checks between liquid tests.
What GH actually measures
GH, general hardness, is the total dissolved calcium and magnesium in your water. These minerals are essential. Fish use them for osmoregulation and bone health, shrimp and snails need calcium to build and molt their shells, and plants draw on magnesium for photosynthesis. Water with very low GH is missing nutrients your livestock depends on, which is why pure RO water is never used straight from the filter.
GH is usually measured in degrees of general hardness (dGH) or in parts per million (ppm), where 1 dGH equals about 17.9 ppm. You can move between these and other units with our aquarium unit converter.
What KH measures and why it buffers pH
KH, carbonate hardness, measures the carbonates and bicarbonates in your water. Think of KH as your pH shock absorber. Acids constantly form in an aquarium from fish waste, the nitrogen cycle, CO2, and driftwood. KH neutralizes those acids before they can lower your pH. As long as KH holds, pH stays steady.
When KH runs out, the buffer is gone, and pH can crash fast and hard. This is the single most important reason to test KH: a sudden pH crash is almost always a depleted KH problem in disguise. Keeping KH at a healthy level is the foundation of the pH stability we cover in our pH guide.
Soft, medium, and hard water at a glance
| Classification | GH (dGH) | GH (ppm) | Best for |
|---|---|---|---|
| Very soft | 0 - 3 | 0 - 50 | Discus, wild bettas, soft-water tetras |
| Soft | 4 - 6 | 50 - 100 | Most tetras, rasboras, dwarf shrimp |
| Medium | 7 - 12 | 100 - 200 | General community tanks |
| Hard | 13 - 18 | 200 - 320 | Livebearers, goldfish |
| Very hard | 19+ | 320+ | African cichlids, brackish setups |
For KH, a practical target for most freshwater tanks is 4 to 8 dKH. Below 3 dKH, you risk pH instability unless you are deliberately running a soft-water tank and watching it closely.
How to raise GH and KH
Raising KH
- Crushed coral or aragonite in the filter or substrate dissolves slowly to add carbonates and buffer pH.
- Baking soda (sodium bicarbonate) raises KH directly; add small amounts and retest.
- Commercial KH buffers give precise control for planted and cichlid tanks.
Raising GH
- A GH remineralizer with calcium and magnesium is the go-to when using RO or RODI water.
- Crushed coral raises both GH and KH, making it a simple all-in-one for many keepers.
- Wonder shell or mineral blocks slowly release minerals for shrimp tanks.
How to lower GH and KH
The cleanest way to soften water is to mix in RO, RODI, or distilled water, which dilutes the minerals to a target ratio. Peat moss and catappa leaves lower hardness modestly while tinting the water. Resist the urge to chase very soft water unless a specific species demands it, because stripping out KH removes your pH buffer and invites the swings we keep warning about. Stable, moderately hard water beats soft, unstable water for the vast majority of fish.
If you keep fish on RO water, treat remineralizing as a recipe: start with zero hardness, add a measured remineralizer to hit your GH target, and add a carbonate source for KH. Mix new water to the same recipe every time so each water change keeps parameters consistent.
How hardness ties into the rest of your water
GH and KH do not work alone. KH supports pH, pH affects the nitrogen cycle and ammonia toxicity, and steady hardness keeps everything predictable as nitrate builds and gets reset. If you inject CO2 in a planted tank, KH and pH together tell you your dissolved CO2 level, which you can dial in with our CO2 calculator.
A simple hardness routine
- Test your tap water for GH and KH so you know your starting point.
- Match fish to your water instead of fighting your tap chemistry.
- Keep KH at 4 dKH or above to protect pH stability.
- Mix new water to the same recipe every water change for consistency.
- Retest after any adjustment and change parameters slowly.
Size your water changes with the water change calculator, and explore more chemistry topics on the Water and Care hub.
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Frequently Asked Questions
What is the difference between GH and KH?
GH (general hardness) measures dissolved calcium and magnesium, the minerals fish and invertebrates need for healthy bodies and shells. KH (carbonate hardness) measures carbonates and bicarbonates, which act as a buffer that stops pH from swinging. In short, GH affects fish and plant health, while KH protects your pH stability. They are related but separate, and you should test both because each does a different job in your tank.
Why does KH buffer pH?
KH represents the carbonate and bicarbonate ions in your water, and these ions neutralize acids as they form. When acids from fish waste, CO2, or driftwood enter the water, KH absorbs them before they can lower pH. As long as KH stays adequate, pH holds steady. When KH is depleted, there is nothing left to absorb acids, and pH can crash suddenly, which is why low KH tanks are prone to dangerous swings.
What are good GH and KH levels for a freshwater tank?
For a general community tank, aim for GH around 4 to 12 dGH and KH around 4 to 8 dKH. Soft-water species like discus prefer lower values, while African cichlids and livebearers want harder water with higher GH and KH. Shrimp have specific needs that vary by species. The most important thing is a stable KH of at least 3 to 4 dKH so your pH does not crash between water changes.
How do I raise GH and KH?
To raise KH, add crushed coral, aragonite, or baking soda (sodium bicarbonate), which adds carbonates. To raise GH, use a remineralizer with calcium and magnesium, or crushed coral, which raises both. Keepers using RO or RODI water start with zero hardness and remineralize to a target. Add slowly and retest, because large fast changes stress fish. Crushed coral in the filter is a popular slow-release method that raises both gradually.
How do I lower GH and KH?
The cleanest way to lower hardness is to mix in RO, RODI, or distilled water, which dilutes the minerals. Peat moss and catappa leaves can also reduce hardness modestly while releasing tannins. Avoid chasing very soft water unless a specific species needs it, because low KH removes your pH buffer and makes the tank unstable. Most fish do better in moderately hard, stable water than in very soft water that swings.
Does low KH cause a pH crash?
Yes, low KH is the leading cause of pH crashes. As fish produce waste and the nitrogen cycle generates acids, those acids consume KH over time. Once KH falls near zero, pH can plummet within hours, killing fish and stalling beneficial bacteria. Testing KH regularly and topping it up with water changes or crushed coral prevents this. A pH crash is almost always a KH problem in disguise.
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