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parameter°dGH

General hardness (GH)

GH is the parameter most closely tied to whether a species is a 'soft water' or 'hard water' animal, and it is quite separate from KH despite the two often moving together. It is reported in German degrees (°dGH) or in ppm as calcium carbonate; 1 °dGH = 17.85 ppm.

In one sentence

General hardness measures dissolved calcium and magnesium. It matters biologically — fish and invertebrates use those ions for osmoregulation, and shrimp and snails need calcium to build shell and exoskeleton.

How to read a measurement

Bands are guidance for interpreting a reading, not thresholds with sharp edges. Sensitivity varies by species and by how long the exposure lasts.

BandRangeVerdictWhat it means
Very soft≤ 3watchSuits blackwater species. Too low for shrimp to moult reliably and for most livebearers.
Soft3–8acceptableSuits most South American and South-east Asian species.
Moderately hard8–15targetSuits the widest range, including shrimp and snails.
Hard15–25acceptableSuits livebearers, rift-lake cichlids and most snails.
Very hard≥ 25watchRift lake territory. Soft-water species do not do well here long term.

Why invertebrates care more than fish

A shrimp builds a new exoskeleton at every moult and a snail grows shell continuously, and both draw calcium from the water. In very soft water shrimp fail to complete moults — the condition keepers call the 'white ring of death' — and snail shells pit and erode. A GH around 6–8 °dGH is a common working minimum for Neocaridina, and nerite snails do better higher still.

What general hardness and carbonate hardness each measureTwo columns side by side. The left column, general hardness, counts calcium and magnesium ions, and matters for osmoregulation, shell and exoskeleton building, and egg development. The right column, carbonate hardness, counts carbonate and bicarbonate ions, and matters for buffering capacity — how far the pH will move when acid is added. A note beneath records that the two usually rise and fall together in tap water, which is why they are confused, and that they can be separated deliberately.GHGeneral hardnessCa²⁺ and Mg²⁺OsmoregulationShell and exoskeletonEgg developmentWhat the animal responds toKHCarbonate hardnessCO₃²⁻ and HCO₃⁻Buffering capacityHow far pH will moveCO₂ / pH relationWhat the water responds toThey usually rise and fall together in tap water, which is most of why they get confused.They can be separated: RO water remineralised with calcium chloride raises GH and not KH.A tank with low KH and a fish that needs high GH is a real and common combination.
GH and KH are quoted in the same unit family and measure different things. GH is what the animal's physiology responds to; KH is what stops the pH moving. Source: AquariumHQ water chemistry model

Changing it

  • To lower GH: dilute with reverse-osmosis or distilled water. Nothing else works reliably.
  • To raise GH: use a purpose-made remineralising salt, or add calcium and magnesium sources such as crushed coral or a GH booster.
  • Changing GH also changes what happens to pH, because the mineral sources involved usually carry carbonate too.

How it is measured

  • Titration test kit (drops until colour change)
  • Test strips
  • Local water supplier's published hardness figure

Typical aquarium range

Typical
4–15 °dGH

Common misconceptions

Often saidWhat is actually the case
GH and KH are the same thing.GH is calcium and magnesium; KH is carbonate and bicarbonate. Water can be high in one and low in the other. Remineralising with the wrong product raises the wrong one.