GH (general hardness)

Total dissolved calcium and magnesium in water, measured in dGH (German degrees) or ppm. Soft water = low GH (<5 dGH); hard water = high GH (>12 dGH). Most community fish tolerate 5-15 dGH.

General hardness (GH) measures the concentration of multivalent cations (mostly Ca²⁺ and Mg²⁺) in water. It's distinct from alkalinity (KH), though the two often correlate in tap water.

GH affects fish osmotic regulation, plant nutrient uptake, and shrimp molting. Some species are hardness-strict: shrimp need adequate calcium for shell formation; African cichlids prefer hard water; soft-water tetras prefer the opposite.

For most beginners, GH is one of the parameters that's easiest to leave alone — match your tap water's GH and pick fish that tolerate it.

Beginner-safe rule

Beginner-safe rule: test GH from your tap. 1 dGH ≈ 17.9 ppm. If GH is 5-15 dGH, most beginner-friendly community fish (tetras, corys, guppies, mollies) will be fine.