KH (carbonate hardness)
KH measures the ability of water to resist changes in pH. In practice it indicates how stable the system is from a chemical point of view.
Detailed explanation
Carbonate hardness represents the concentration of carbonates (CO3) and bicarbonates (HCO3) dissolved in water. These compounds work like a natural buffer: they absorb changes in acidity without causing the pH to suddenly collapse.
In a marine aquarium KH plays an even more critical role because corals consume carbonates to build their skeleton. If the KH drops too much, the pH becomes unstable, the corals stop calcifying and the whole system suffers within a few hours.
In tropical freshwater aquariums the situation changes a little. Many Amazon fish prefer soft waters with low KH. But even here a minimum amount of stability is needed: a KH below 2 dKH means that the pH can oscillate dangerously between day and night.
For outdoor ponds KH is essential especially in spring when the algae starts again. Unicellular algae consume CO2 during the day, causing the pH to rise. If the KH is too low the pH can reach above 9.5 in the evening with serious consequences for the koi carp.
In brackish aquariums, a medium-high KH is needed to maintain the slightly alkaline pH that estuarine species require.
Reference values by aquarium type
| Aquarium type | Recommended range |
|---|---|
| Marine (reef) | 7 - 12 dKH |
| Tropical freshwater | 3 - 8 dKH |
| Coldwater / pond | 4 - 12 dKHIn koi ponds it is recommended to stay above 5 dKH to avoid pH fluctuations during the day |
| Brackish | 8 - 14 dKH |
How it is measured
The drop test is the most widespread method among aquarists. Salifert produces a widely used kit for marine aquariums. For freshwater aquariums, JBL or Sera kits are also available. Drops of reagent are added to the water sample until the colour changes. Each drop corresponds to 1 dKH. For a more precise digital reading you can use a photometer specific for alkalinity. The ICP-OES does not directly measure KH.
What happens when it is too low
The pH becomes unstable with oscillations of up to 0.3-0.5 points in a few hours. SPS corals stop growing. Fish are showing signs of respiratory distress. In ponds the pH at night can drop below 7 while during the day it exceeds 9. In extreme cases there is a risk of sudden death.
What happens when it is too high
Rarely dangerous in itself but can cause calcium precipitation (calcium snow). The water becomes milky. In freshwater aquariums, too high a KH makes it impossible to lower the pH for species that require acidic water.
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