GH, or general hardness, measures the dissolved calcium and magnesium in your water — in everyday terms, whether your water is “hard” or “soft”. It matters because different fish and invertebrates evolved for different hardness, and shrimp in particular need adequate GH to moult and survive.

Here is what GH is and what level your livestock needs.

What GH does

GH reflects the mineral content fish and shrimp use for biological functions — bone, shell and osmotic regulation. It is measured in degrees (dGH) or ppm, and is separate from KH (carbonate hardness), which buffers pH.

UK water hardness varies dramatically by region — London and the South East are very hard, while Scotland, Wales and the South West are soft.

Ideal GH by livestock

• Community fish: 4–12 dGH suits most.
• Soft-water species (tetras, discus, wild bettas): 2–8 dGH.
• Hard-water species (mollies, guppies, African cichlids): 12–20 dGH.
• Cherry shrimp: 6–15 dGH — essential for successful moulting.

How to adjust GH

To raise GH: add a mineral supplement like Seachem Equilibrium or Salty Shrimp GH+, or use mineral-rich substrate.

To lower GH: dilute with RO water. Check your regional hardness on your water company’s website, and test with an API GH/KH kit.

Frequently asked questions

What is GH in an aquarium?

GH, or general hardness, measures the dissolved calcium and magnesium in your water — essentially how hard or soft it is. Fish and shrimp need appropriate GH for healthy biological function.

What should GH be in a fish tank?

4–12 dGH suits most community fish. Soft-water species prefer 2–8 dGH, hard-water species 12–20 dGH, and cherry shrimp 6–15 dGH for proper moulting.

How do I raise GH in my aquarium?

Add a mineral supplement such as Seachem Equilibrium or Salty Shrimp GH+, or use mineral-rich substrate. Test before and after with a GH/KH kit.

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— Jaqline Smalling, FishForever