dKH
Carbonate hardness (KH)
What is it
Carbonate hardness (KH, Karbonathärte) measures dissolved carbonates (CO₃²⁻) and bicarbonates (HCO₃⁻). These ions form a natural buffer that resists pH changes.
KH is measured in German degrees (dKH). One dKH equals 17.8 mg/L of calcium carbonate. Tap water commonly has KH 5-15, while reverse-osmosis water has KH 0.
GH and KH are independent: water can have high GH and low KH, although that is rare, or low GH and high KH. In active-soil tanks, KH is consumed over time as the soil exchanges hydrogen ions with bicarbonates.
Why it matters
KH is pH's life insurance. Without enough carbonate buffer, pH can crash overnight as fish, plants and bacteria respire and produce CO2 without photosynthesis to offset it. This acid crash can kill an entire population within hours.
In injected-CO2 tanks, KH determines how much pH falls: the CO2/KH/pH relationship follows a precise table. At KH 4 and pH 6.6, CO2 is about 30 mg/L, which is ideal; at KH 1 and pH 6.6, CO2 would be only 7 mg/L.
For Caridina shrimp on active soil, KH must be 0-1 because the soil provides an alternative acidic buffer. Adding KH is counterproductive: the soil consumes it anyway and wears out faster.
Ideal ranges
Interactions with other parameters
KH is pH's shield. CO2 injection into water with KH 4 lowers pH to 6.6; if KH were 10, three times as much CO2 would be needed to reach the same pH.
| Tank type | Min | Max | Unit |
|---|---|---|---|
| Tropical community | 3 | 10 | dKH |
| Planted high-tech | 2 | 5 | dKH |
| Planted low-tech | 3 | 12 | dKH |
| Shrimp tank | 0 | 2 | dKH |
Out of range: what happens
KH that is too low (below 2 dKH without active soil) brings a real risk of an overnight acid crash, unstable pH swings of 1+ point through the day, and stressed nitrifying bacteria.
KH that is too high (above 10 dKH) makes pH difficult to lower with CO2, because a dangerous amount would be needed; it produces very alkaline water and limits species choice to hard-water-tolerant animals.
Rapid KH use (a fall of 2+ dKH per week) signals that active soil is consuming buffer or that organic acids are excessive. Monitor it and supplement where necessary.
Common Myths
- Low KH is dangerous; this is false for planted tanks with active soil, which stabilizes pH even at KH 0.
- You can lower KH with vinegar or lemon; this only creates a temporary crash and pollutes the water.
Measurement and adjustment
How to measure
Use a liquid titration reagent test, with the same procedure as a GH test: add drops until the colour changes, typically from blue to yellow. The number of drops equals dKH.
Testing KH and pH together lets you calculate dissolved CO2 with a CO2/KH/pH table. This is essential for anyone using pressurized CO2.
In active-soil tanks, test KH weekly for the first 8 weeks. The rapid initial consumption indicates the soil's remaining capacity.
How to adjust
To raise KH, use sodium bicarbonate (NaHCO₃): 1 gram per 50 litres raises KH by about 1 dKH. Dissolve it in separate water and add it slowly. Commercial products such as Seachem Alkaline Buffer are more controllable.
To lower KH, mix reverse-osmosis water with tap water, or use active soil that naturally consumes KH. Peat in the filter lowers both KH and pH.
For CO2 tanks, keep KH 3-5 for a good compromise between pH stability and the ability to reach 30 mg/L CO2 at a safe pH of 6.4-6.8.
Pro Tips
- When using active soil such as ADA Amazonia, do not add KH-raising products: the soil absorbs it all and exhausts its buffering capacity months early.
- KH and pH are an aquascaper's best friends: use their values on the Tillmans table to calculate exactly how much CO2 is in the tank.