Last Updated: June 8, 2026
Water hardness is one of the most misunderstood parameters in the hobby, yet it quietly shapes whether your fish thrive or merely survive. Two related but distinct measurements, GH (general hardness) and KH (carbonate hardness), determine which species suit your water, how stable your pH stays, and how well your plants grow. Many beginners obsess over pH while ignoring the hardness values that actually drive it. This guide explains GH and KH in plain language, the units you will see on test kits, how hardness affects fish and plants, and exactly how to raise or lower each one safely.
GH vs KH: What They Actually Measure
It is easy to confuse the two, but they measure different things. General hardness (GH) measures the total concentration of dissolved minerals, primarily calcium and magnesium. This is the “hardness” most people mean when they call water hard or soft. It matters to fish and invertebrates because these minerals support biological functions like osmoregulation and shell or bone development.
Carbonate hardness (KH) measures carbonates and bicarbonates, which act as a buffer. KH is your tank’s defense against pH swings: a higher KH resists changes in pH, while a low KH lets pH crash unexpectedly. You can think of KH as the stability cushion for your water chemistry. The two are related but independent; you can have water that is high in one and low in the other. To measure both reliably, you need a proper test kit, as covered in our aquarium water testing guide.
Units: dGH, dKH, and ppm
Hardness is reported in two common unit systems, and test kits vary, so it helps to know both:
- Degrees (dGH / dKH): German degrees of hardness, the most common hobby unit. One degree equals roughly 17.9 ppm.
- ppm (parts per million) or mg/L: a direct concentration measure. Many liquid test kits and digital meters report in ppm.
As a rough guide, water around 0-3 dGH is very soft, 4-8 dGH is soft to moderately soft, 8-12 dGH is moderately hard, and above 12 dGH is hard. Always note which unit your kit uses so you do not misread a result by a factor of nearly eighteen.
How Hardness Affects Fish, Plants, and pH
Effect on Fish
Fish evolved in specific water conditions, and matching them reduces stress and disease. Soft-water species such as many tetras, discus, and dwarf cichlids prefer low GH, while hard-water species such as African cichlids, livebearers (guppies, mollies, platies), and goldfish need higher mineral content. Keeping a hard-water fish in very soft water, or vice versa, leads to chronic stress and poor longevity even if it does not kill quickly.
Effect on Plants
Plants draw on calcium and magnesium from GH for healthy growth, so extremely soft water can cause deficiencies. Most popular aquarium plants, however, tolerate a wide range. Our planted tank guide covers how to support plant nutrition across different water types.
Effect on pH Stability
This is where KH becomes critical. A healthy KH buffers your water, keeping pH steady between water changes. When KH is too low, pH can swing or crash suddenly, which is dangerous for fish and can stall your nitrogen cycle. If you struggle with unstable pH, the cause is very often low KH rather than the pH itself.
How to Raise GH and KH
Raising hardness adds minerals or buffering capacity. Common, reliable methods include:
- Crushed coral: Added to the substrate or filter, it slowly dissolves to raise both GH and KH, ideal for hard-water species like African cichlids. See our cichlid aquarium setup guide.
- Aragonite or limestone rock: Naturally buffers and hardens water over time.
- Wonder Shell and mineral blocks: Dissolvable mineral supplements that add calcium and other minerals.
- Remineralizing additives: GH/KH boosters formulated for aquarium use, dosed to a target value.
- Baking soda (sodium bicarbonate): Raises KH specifically; use cautiously and in small amounts.
Hard-water fish like those in our goldfish tank setup guide benefit from a stable, mineral-rich environment, so these methods help keep them healthy.
How to Lower GH and KH
Softening water means removing or diluting minerals. The safest approaches are:
- Reverse osmosis (RO) water: RO strips out nearly all minerals; you blend it with tap water to dial in a target hardness. This is the most precise method for soft-water species.
- Peat moss or peat filtration: Releases tannins and acids that gently lower GH and KH, often tinting the water amber.
- Indian almond leaves and driftwood: Release tannins that mildly soften and acidify water, popular for bettas and soft-water biotopes.
- Rainwater or distilled water (with care): Naturally soft, but must be remineralized to a baseline and tested for purity.
Whenever you alter hardness, do it gradually. Sudden swings stress fish more than a stable, slightly imperfect value. Make changes over several days and test frequently.
Matching Hardness to Your Stock
| Water Type | Approx. GH | Example Species | How to Achieve |
|---|---|---|---|
| Very soft | 0-4 dGH | Discus, many tetras, wild bettas | RO water, peat, almond leaves |
| Moderate | 4-12 dGH | Most community fish, many plants | Conditioned tap water |
| Hard | 12+ dGH | African cichlids, livebearers, goldfish | Crushed coral, mineral blocks |
The simplest path for beginners is to choose fish that suit your tap water rather than constantly fighting your source water. Test your tap GH and KH first, then build your stocking plan around it. For the bigger picture on getting a tank started correctly, see our complete fish tank setup guide.
Frequently Asked Questions
What is the difference between GH and KH in simple terms?
GH measures total dissolved minerals (mainly calcium and magnesium) that fish and plants need, while KH measures the carbonate buffer that keeps your pH stable. GH is about mineral content; KH is about pH stability.
Does hardness change my pH?
Indirectly, yes. KH buffers pH, so harder, well-buffered water tends to hold a stable, often higher pH, while low-KH soft water allows pH to drop or swing. If your pH is unstable, check your KH first.
Can I use crushed coral and RO water together?
You typically choose one direction. Crushed coral raises hardness for hard-water species, while RO water lowers it for soft-water species. Mixing tap and RO water lets you target a precise middle hardness, but adding crushed coral would push it back up.
Should I adjust my water or pick fish that match it?
For most beginners, choosing fish suited to your existing tap water is far easier and safer than constantly adjusting hardness. Test your tap GH and KH and stock accordingly. Adjust only when keeping a species with specific needs.
How often should I test GH and KH?
Test when setting up a tank, when introducing new species, and periodically thereafter, especially if you adjust hardness or notice pH instability. Our water testing guide explains a sensible testing schedule.
