Understanding GH and KH in Aquariums
Water hardness is one of the most misunderstood parameters in the aquarium hobby. Two separate measurements — GH (general hardness) and KH (carbonate hardness / alkalinity) — are often conflated, but they measure fundamentally different properties of water and affect fish in different ways.
General Hardness (GH): Calcium and Magnesium Content
General hardness measures the total concentration of dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions in water. These ions are responsible for water's "hardness" in the household sense — they cause limescale buildup in kettles and pipes.
For aquatic organisms, GH affects osmoregulation — the biological process by which fish and invertebrates maintain internal ion balance. Fish adapted to soft, ion-poor water (tetras, discus, wild bettas) have osmoregulatory systems tuned to extract the small amounts of ions available in their native environment. Placing them in hard water forces their kidneys and gills to work harder to excrete excess ions, causing chronic stress.
For invertebrates — shrimp, snails, crabs — GH is critical for exoskeleton development and molting. Neocaridina shrimp (cherry shrimp) require GH of 6–8 dGH as a minimum for successful molts. Below 4 dGH, failed molts ("death by molt") become common, with animals found stuck in their old exoskeleton unable to complete the process.
Carbonate Hardness (KH): pH Buffering
Carbonate hardness — also called alkalinity or buffering capacity — measures the concentration of carbonate (CO₃²⁻) and bicarbonate (HCO₃⁻) ions. KH does not directly affect fish health at typical aquarium levels, but it has a critical indirect effect: KH determines how stable your pH is.
In a planted tank or any tank with CO₂ fluctuation, pH rises overnight (CO₂ consumed by plants during the day, no CO₂ production at night). The magnitude of this swing is directly controlled by KH:
- KH 0–2 dKH: pH can swing 2–3 points overnight — potentially lethal
- KH 3–5 dKH: Moderate swing (0.5–1 pH unit) — manageable for most species
- KH 6+ dKH: Very stable pH — minimal overnight variation
This creates a dilemma for soft-water species that need low KH for accurate pH: low KH means unstable pH, but the instability itself is harmful. The solution is either CO₂ injection to control pH directly, or very dense plant growth to minimize CO₂ fluctuation.
Unit Conversions Explained
The relationship between hardness units is fixed by the molecular weight of CaCO₃:
- 1 dGH (German degree of hardness) = 17.848 ppm (mg/L as CaCO₃) = 0.17848 mmol/L
- 1 dKH uses the same conversion factor as dGH (both reference CaCO₃)
- ppm vs mg/L: In aquarium context, these are equivalent for water at normal aquarium density
Different test kits and water reports use different units — German kits typically report dGH/dKH, US municipal water reports use mg/L (ppm), and scientific literature uses mmol/L. This converter handles all three directions.
RO Water and Remineralization
Reverse osmosis (RO) water has GH and KH of approximately zero, which is ideal for adjusting water to specific target parameters. RO water is mixed with tap water to dilute hardness, or remineralized using mineral additives:
- Caridina shrimp (Crystal, Taiwan Bee): SL-Aqua Purify or Salty Shrimp Bee Shrimp Mineral GH+ to achieve GH 4–6, KH 0–1
- Neocaridina shrimp (Cherry, Blue Velvet): Salty Shrimp GH/KH+ to achieve GH 6–8, KH 2–4
- Blackwater soft water (discus, wild bettas): RO/tap blend to achieve GH 1–5, KH 0–3
- Marine/reef: Specialized reef salt mixes to achieve NSW parameters