KH
KH — Carbonate Hardness (Alkalinity)
What is it?
KH (Karbonathärte in German) measures the concentration of carbonates and bicarbonates in the water. It acts as a** pH buffer**: the higher the KH, the more stable the pH is in the face of changes.
Units: German degrees (°dKH) or ppm of CaCO3 (1 °dKH = 17.9 ppm)
KH–pH Relationship
CO2 interacts with KH to determine the pH. If the KH is very low (<2 °dKH), the pH can vary dramatically between day and night (as plants absorb CO2 at night).
KH/CO2/pH Reference Table (approximate values)
| KH (°dKH) | pH 6.5 | pH 7.0 | pH 7.5 |
|---|---|---|---|
| 1 | 6 ppm CO2 | 3 ppm CO2 | 1 ppm CO2 |
| 4 | 22 ppm CO2 | 11 ppm CO2 | 4 ppm CO2 |
| 8 | 45 ppm CO2 | 22 ppm CO2 | 7 ppm CO2 |
Recommended Values
| Type | Recommended KH |
|---|---|
| Planted Aquarium with CO2 | 3–8 °dKH |
| General Freshwater | 4–10 °dKH |
| African cichlids | 8–15 °dKH |
| Marine | 8–12 °dKH (expressed as Alk in meq/L or dKH) |
| Neocaridina Shrimp | 4–8 °dKH |
| Caridina Shrimp | 0–2 °dKH (remineralized osmotized water) |
How to Adjust the KH
Increase KH:
- Sodium bicarbonate (NaHCO₃): 0.7 g raises dKH by ~1° in 100 L
- Potassium carbonate (K2CO3): also provides potassium (useful for planted tanks)
- Limestone (a slow but steady process)
- Tap water with high KH
Lower KH:
- Reverse osmosis + controlled remineralization
- Silt (slow-acting, also lowers pH)
- Mix osmotized water with tap water