Aquarium Water Hardness Chart (GH & KH)
Convert dGH and dKH to ppm and soft, medium, or hard categories, plus ideal GH and KH ranges for popular aquarium fish. A complete water hardness reference chart.
Water hardness decides which fish will thrive in your tank and whether your pH stays stable. It comes in two parts: GH, the general hardness from calcium and magnesium, and KH, the carbonate hardness that buffers pH. Test kits report these in degrees (dGH, dKH) or ppm, which makes care sheets confusing. This chart converts both and maps them to the soft, medium, and hard categories fish profiles use.
Quick answer: Multiply degrees by 17.86 to convert dGH or dKH to ppm. Water is soft below about 6 dGH (107 ppm), medium hard from 7 to 11 dGH (125 to 196 ppm), and hard above 12 dGH (214 ppm). Most community fish do well at 4 to 12 dGH with a KH of 3 to 8 dKH for stable pH. Keep KH at 3 dKH or higher to prevent pH crashes. Learn the science in our GH and KH explained guide and convert any reading with the aquarium unit converter.
Water hardness conversion chart (degrees to ppm)
One degree of hardness (dGH or dKH) equals 17.86 ppm of calcium carbonate. The categories below describe general hardness (GH), the value most often quoted in fish care sheets.
| Degrees (dGH / dKH) | ppm (mg/L CaCO3) | Hardness classification |
|---|---|---|
| 1 | 17.9 ppm | Very soft |
| 2 | 35.7 ppm | Very soft |
| 3 | 53.6 ppm | Very soft |
| 4 | 71.4 ppm | Soft |
| 5 | 89.3 ppm | Soft |
| 6 | 107.2 ppm | Soft |
| 7 | 125.0 ppm | Medium hard |
| 8 | 142.9 ppm | Medium hard |
| 9 | 160.7 ppm | Medium hard |
| 10 | 178.6 ppm | Medium hard |
| 11 | 196.5 ppm | Medium hard |
| 12 | 214.3 ppm | Hard |
| 13 | 232.2 ppm | Hard |
| 14 | 250.0 ppm | Hard |
| 15 | 267.9 ppm | Hard |
| 16 | 285.8 ppm | Hard |
| 17 | 303.6 ppm | Hard |
| 18 | 321.5 ppm | Very hard |
| 20 | 357.2 ppm | Very hard |
| 25 | 446.5 ppm | Very hard |
| 30 | 535.8 ppm | Very hard |
Ideal GH and KH ranges for popular fish
These are typical comfortable ranges for healthy keeping. Stable hardness matters more than hitting an exact number, so choose fish that suit your tap water where possible. pH is included because it tracks closely with KH.
| Fish / species | GH (dGH) | KH (dKH) | pH |
|---|---|---|---|
| Discus | 1 to 4 | 0 to 3 | 6.0 to 7.0 |
| Neon & cardinal tetra | 1 to 6 | 1 to 5 | 5.5 to 7.0 |
| Angelfish | 3 to 8 | 3 to 8 | 6.5 to 7.5 |
| Betta | 3 to 12 | 3 to 8 | 6.5 to 7.5 |
| Corydoras catfish | 2 to 12 | 2 to 8 | 6.0 to 7.5 |
| Cherry & neocaridina shrimp | 4 to 8 | 1 to 6 | 6.5 to 7.5 |
| Guppy | 8 to 12 | 5 to 12 | 7.0 to 8.0 |
| Platy & swordtail | 10 to 20 | 6 to 12 | 7.0 to 8.2 |
| Molly | 12 to 25 | 6 to 14 | 7.5 to 8.5 |
| Goldfish | 5 to 19 | 4 to 12 | 7.0 to 8.4 |
| African Rift Lake cichlid | 10 to 20 | 10 to 18 | 7.8 to 8.6 |
How to use these numbers
Start by testing your tap water for both GH and KH. Once you know what comes out of the faucet, match it to fish in the same range rather than fighting your water chemistry. Soft-water tap suits tetras, discus, and angelfish; hard-water tap suits livebearers and Rift Lake cichlids. This single decision saves more fish than any chemical you can buy.
KH is the value to watch for stability. Because it buffers pH, a KH below about 3 dKH leaves the tank vulnerable to a pH crash as waste acids accumulate. If your readings drift low, raising KH with a carbonate buffer or crushed coral protects the whole system. Read the full mechanism in our GH and KH explained guide.
Adjusting hardness safely
If you must change hardness, do it slowly and aim for stability over perfection. To soften water, mix in RO or RODI water to dilute the minerals rather than dosing acids, which cause swings. To harden water, add a calcium and magnesium supplement for GH and a carbonate buffer or crushed coral for KH. Change values gradually across several days so fish can adjust, and test after each step.
Remember that hardness, pH, and your other parameters are linked. A sudden hardness change can move pH, so monitor both. For day-to-day testing rhythm and how hardness fits with ammonia, nitrite, and nitrate, see our water chemistry guides, and convert any reading between degrees and ppm with the unit converter.
More water reference tools
For target parameters by species, see our water parameters by species chart, and for the relationship between CO2, pH, and KH in planted tanks, see the CO2 / pH / KH chart. Every quick-lookup table lives in the reference charts hub.
Frequently Asked Questions
What is the difference between GH and KH?
GH (general hardness) measures dissolved calcium and magnesium, the minerals fish and shrimp need for healthy bones, scales, and molting. KH (carbonate hardness) measures carbonates and bicarbonates, which buffer the water and stop pH from crashing. GH affects which fish thrive, while KH keeps pH stable. They often rise and fall together but they are separate measurements, and you should test both.
How do I convert dGH or dKH to ppm?
Multiply degrees by 17.86 to get ppm (mg/L of calcium carbonate). So 4 dGH is about 71 ppm, 8 dGH is about 143 ppm, and 12 dGH is about 214 ppm. To go the other way, divide ppm by 17.86 to get degrees. Test kits report either degrees or ppm, so this conversion lets you compare a reading against any fish care sheet regardless of the unit it uses.
What water hardness do most community fish need?
Most common community fish are comfortable in soft to moderately hard water, roughly 4 to 12 dGH (about 70 to 215 ppm) with a KH of 3 to 8 dKH for stable pH. Hardy species like guppies, platies, and mollies prefer the harder end, while tetras, rasboras, and many catfish prefer the softer end. Matching your tap water to fish that suit it is far easier than constantly adjusting hardness.
Why does low KH cause pH crashes?
KH is the water buffer. Carbonates neutralize the acids that build up from fish waste, leftover food, and biological filtration. When KH is very low, usually under 3 dKH, those acids overwhelm the buffer and pH can drop suddenly and dangerously, a "pH crash" that stresses or kills fish. Keeping KH at 3 dKH or higher gives the tank enough buffering to hold pH steady between water changes.
Should I soften hard water for my fish?
Usually not, unless you are keeping soft-water specialists like discus or wild caught tetras and breeding them. Stable hardness matters more to fish than a perfect number, and chasing a target with chemicals causes swings that do more harm than the original hardness. If you need soft water, mix in RO or RODI water rather than dosing acids. For most fishkeepers, choosing fish that suit your tap water is the wiser path.
How do I raise GH and KH for African cichlids?
Rift Lake cichlids need hard, alkaline water, roughly 10 to 20 dGH and 10 to 18 dKH with a pH of 7.8 to 8.6. Raise GH with a calcium and magnesium mineral supplement, and raise KH with a carbonate buffer or crushed coral and aragonite substrate, which dissolve slowly to hold the water hard and alkaline. Test both values and adjust gradually so the tank stays stable rather than swinging.
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