What carbonate hardness actually does
Carbonate hardness (KH) measures your water's buffering capacity — how well it cushions acid inputs without the pH immediately tipping over. Chemically, it's mainly driven by bicarbonate ion content (HCO3⁻).
Think of KH as a shock absorber for your pH: the higher the KH, the more stable your pH stays even under acid input (e.g. from CO₂ injection or organic breakdown). Once KH is depleted, the water loses this buffering entirely — and the pH can crash drastically within a few hours.
Ideal range by stocking
| Stocking / tank type | KH range | Note |
|---|---|---|
| General community tank | 4 – 8 °dKH | Standard range |
| Discus, Amazon species (soft water) | 1 – 4 °dKH | low KH is intentional, but watch crash risk |
| Malawi/Tanganyika cichlids | 10 – 18 °dKH | hard, strongly buffered water desired |
| Planted tank with CO₂ injection | 3 – 6 °dKH | lower KH makes pH-lowering via CO₂ easier |
| Shrimp (Neocaridina) | 4 – 8 °dKH | stability matters more than the exact number |
Why KH fluctuates
The most common causes of falling carbonate hardness:
Natural consumption over time: Organic acids from fish waste and leftover food continuously consume carbonate hardness.
Soft tap water or RO water: Using water with very low KH for water changes gradually lowers overall KH in the tank.
Tannins from wood and leaves: Driftwood, alder cones, or leaf litter release acids that further deplete KH.
Very soft substrates: Some aquasoil substrates for planted tanks actively and intentionally lower KH — worth being aware of.
What to do about deviations
If KH is too low (crash risk): dedicated KH-boosting products or sodium bicarbonate (baking soda, dosed very carefully) raise buffering capacity directly. Crushed coral or aragonite in the filter also release KH continuously and gently.
If KH is too high (e.g. a soft-water fish tank that's over-buffered): mixing in RO or rainwater during water changes lowers KH gradually. Peat in the filter also has a lowering effect.