General hardness (GH)

GH (general hardness) is the dissolved calcium and magnesium in your water — the minerals shrimp harden new shells with, snails build spires from and fish regulate their body chemistry against. A general community tank is comfortable at 6–12 °dGH; soft-water species breed at 2–6 °dGH, hard-water livebearers and rift-lake cichlids want 8–18 °dGH or more. One degree, 1 °dGH, equals 17.86 ppm CaCO₃.

What is general hardness (GH)?

General hardness measures the total concentration of divalent cations in the water — practically, calcium (Ca²⁺) and magnesium (Mg²⁺). It's measured in degrees German hardness (°dGH); 1 °dGH ≈ 17.86 ppm CaCO₃, which works out to about 7.1 mg/L of calcium if calcium carried it alone — chalk-aquifer tap water splits it roughly 4:1 between calcium and magnesium. GH and KH measure different things — the GH vs KH guide untangles them.

What GH should you aim for?

For a general community tank, 6–12 °dGH. Beyond that, GH has no single ideal — you set it to the livestock. Hard-water species (livebearers, African cichlids, most snails and shrimp that build shells or exoskeletons) want GH above 8 °dGH for healthy moults and shells. Soft-water species (most South American tetras, discus, wild Apistogramma, crystal and bee shrimp) breed reliably only below 6 °dGH.

Tank type	GH target
Community tank	6–12 °dGH
Neocaridina (cherry shrimp)	6–12 °dGH
Caridina (bee and crystal shrimp)	4–6 °dGH
Livebearers, rift-lake cichlids, snails	8–18+ °dGH
Discus, wild Apistogramma, breeding tetras	2–6 °dGH

Note the caridina row: they run their KH near zero, but they still need measurable GH — the calcium for each moult has to come from the water column. The shrimp-safe parameters guide has the species-by-species numbers.

Why GH matters

GH is the parameter livestock care about most after temperature. It drives moulting in shrimp and shell-building in snails, and it sets whether soft-water fish will colour up and spawn at all.

Plants count too: calcium and magnesium are macronutrients, and a very soft tank can starve them. Stunted, twisted new tips point at calcium; yellowing between the veins on older leaves points at magnesium. Both deficiencies show up long before the fish or shrimp complain.

GH and KH are independent: tap water from a chalk aquifer is high in both, but an RO + Equilibrium remineralised tank can have any combination. The hardness converter translates between °dGH, ppm, and the Ca/Mg breakdown your test kit may report.

Why GH drifts on its own

Evaporation is the quiet one: water leaves, minerals stay. Top up with tap and you're adding fresh minerals on top of the old ones, so GH in an open tank ratchets upward month over month — a degree or two per month is common in summer. Top off with RO or distilled water instead, and the number stops creeping.

Hardscape pulls the same direction. Limestone, seiryu and other calcareous stone (anything that fizzes under a drop of vinegar) dissolves slowly, fastest in soft, acidic water — exactly the aquascape where you least want it. Crushed coral in the substrate does it on purpose. In the other direction, active soils exchange calcium away, and a dense shrimp or snail population is a genuine mineral drain. A monthly test tells you which way your tank leans.

How to test GH

Drop tests, counting drops to the colour change. Test water at source (tap or RO mix) and in the tank — substrate and rocks can shift GH over time. Once a month is usually enough in a stable tank.

Each drop of reagent stands for 1 °dGH; double the sample volume and each drop counts 0.5, which matters in a breeding tank. And a TDS meter is not a GH test — it sums everything ionic, fertiliser and nitrate included, so two tanks with identical TDS can carry very different GH. Your water utility publishes a hardness figure too — a decent first guess, but it's the hardness at the works, not in your tank after months of top-offs and hardscape.

Signs GH is too low — or too high

Below 3 °dGH in a tank with calcium-hungry inhabitants: shrimp moult-fail, snail shells pit and erode, fish develop fin curl from the underlying calcium deficiency. Above 18 °dGH in a soft-water tank: Apistogramma fail to spawn, soft-water tetras stop colouring up.

The signature failure at the low end is the shrimp keeper's white ring of death — a pale band behind the head where the old shell should have separated cleanly from the new one. A shrimp showing it is usually past saving, and the cause is almost always GH that's too low or too unstable. At the high end the failure is mechanical: the eggs of soft-water species harden in mineral-rich water and never develop, which is why a pair of Apistogramma can court endlessly in 15 °dGH without a single spawn hatching.

How to raise GH

Dose a remineraliser (Salty Shrimp GH/KH+, Seachem Equilibrium, JBL Aquadur) — these add calcium and magnesium in the right ratio. Remineralising RO water to a known GH is more reliable than chasing the number in an established tank.

The product class matters more than the brand: "GH/KH+" salts raise both hardnesses, "GH+"-only salts raise just GH — for caridina over active soil, the latter is mandatory. And verify with a test rather than trusting the label's grams-per-litre table; remineraliser salts pull moisture from the air and dosing spoons lie.

How to lower GH

Dilute with RO water; the simplest path is to switch the source water rather than fight the GH downstream. A 50:50 RO/tap mix typically halves your starting GH — the water-change impact tool shows how far a planned change moves the number before you commit.

One trap: a household ion-exchange softener is not an RO unit. It swaps calcium and magnesium for sodium, so the GH test reads soft while the fish get sodium-loaded water — don't fill the tank from the softened line. And move hardness slowly in either direction; shrimp and snails handle a stable "wrong" GH far better than a fast correction.