GH (General Hardness): What It Means and How to Manage It in a Freshwater Tank
GH, or general hardness, is a measurement of the dissolved calcium and magnesium in aquarium water, the two minerals that make water read as “hard” or “soft.” It is a different number from KH: GH tracks calcium and magnesium specifically, while KH tracks carbonate and bicarbonate buffering capacity, and the two can move independently of each other. A tank can have high GH and low KH, or the reverse, so testing only one of them tells you half the picture.
How GH Is Tested and What the Numbers Mean
Most hobbyist test kits measure GH by titration: you add a reagent to a water sample drop by drop and count how many drops it takes for the color to change. That drop count is then multiplied by a conversion factor printed on the kit to give a result in degrees GH (dGH), the most common unit on Indian and European test kits. GH is sometimes also reported in ppm or in milliequivalents per liter (mEq/L). As rough conversions, 1 dGH is about 17.9 ppm of calcium carbonate equivalent, and 1 mEq/L is about 2.8 dGH. There is no single “GH chemical”: it is the sum of two dissolved ions, calcium (Ca2+) and magnesium (Mg2+), read together as one hardness number.
A TDS (total dissolved solids) meter is not a substitute for a GH test. TDS measures everything dissolved in the water, including sodium, chloride, nitrate, and other ions that have nothing to do with hardness, so a TDS reading can rise or fall for reasons that have no bearing on GH at all. If an actual GH figure is needed, a liquid titration kit is the reliable option.
Target GH Ranges by Species and Tank Type
There is no single correct GH for a freshwater tank: it depends entirely on what is living in it. Most community fish, community plants, and general planted tanks do fine across a fairly wide GH 4 to 8 range. Neocaridina shrimp (Cherry Shrimp, Blue Dream, and similar color morphs) tolerate roughly GH 6 to 8 and are forgiving of small swings either side of that. Caridina shrimp (Crystal Red, Crystal Black, Sulawesi species) are far less forgiving and generally need soft, stable water in a narrower GH range, which is why they are usually kept in remineralized RODI water rather than tap water that varies week to week. African cichlids and livebearers sit at the other end of the scale and generally do better in harder water, often GH 10 or above, closer to the mineral content of their native lakes. Our beginner’s shrimp tank setup guide goes through the exact GH and KH targets for Neocaridina versus Caridina in more detail.
Remineralizing RODI or Very Soft Tap Water
Water at or near 0 TDS, whether from an RODI unit or from naturally very soft tap water, has essentially no GH of its own. Adding fish or shrimp to water with no mineral content at all can cause osmotic stress, since their bodies are adapted to regulate against a certain baseline of dissolved minerals. A remineralizer restores that baseline before the water goes anywhere near livestock. Seachem Replenish is built for this in general freshwater tanks: it raises GH using a blend of calcium, magnesium, sodium, and potassium salts, dosed to bring RODI or very soft water back up into a workable range for fish. Shrimp keepers chasing a narrower, species-specific GH and KH target typically reach for a dedicated mineral supplement instead, such as the mineral and salt products in the Shrimp King range, formulated around the tighter tolerances Caridina shrimp need.
Common GH Mistakes
- Testing only pH and assuming it says anything about GH: they are separate measurements and do not move together
- Trusting a TDS meter as a GH reading, when TDS includes minerals and non-mineral dissolved solids alike
- Remineralizing a shrimp tank with a general freshwater conditioner instead of a mineral supplement matched to the species
- Changing GH sharply in one large water change rather than adjusting it gradually, which stresses fish and shrimp more than a stable, slightly imperfect number does
FAQ
What does GH stand for?
General hardness. It measures the combined concentration of dissolved calcium and magnesium in water.
Is there a single chemical that GH measures?
No. GH is not one chemical: it is the sum of two dissolved ions, calcium and magnesium, reported together as one hardness number.
What is a good GH for a freshwater aquarium?
It depends on what’s kept in it. Most community tanks and plants do well around GH 4 to 8, Caridina shrimp usually need softer, more stable water within a narrower band, and African cichlids and livebearers generally prefer harder water above GH 10.
Is GH the same as KH?
No. GH measures calcium and magnesium; KH measures carbonate and bicarbonate buffering capacity. A tank’s GH and KH can be high, low, or mismatched independently of each other.
How is GH measured, in ppm or dGH?
Both units show up on test kits. dGH (degrees GH) is the most common unit on hobbyist kits, and it converts to roughly 17.9 ppm of calcium carbonate equivalent per degree.
What raises GH in an aquarium?
A remineralizer added in a measured dose, such as Seachem Replenish for general freshwater use or a shrimp-specific mineral supplement for Caridina. Crushed coral or certain rocks can also raise GH gradually, though less predictably than a dosed product.
Shop GH Testing and Remineralizing
See Planted Aquarium Test Kits for GH testing, Seachem Replenish for general freshwater remineralizing, and the Shrimp King range for shrimp-specific mineral and salt supplements.