Understanding GH (General Hardness)

GH is the parameter aquarists ignore until they kill a tank of shrimp. I learned this the hard way. My first attempt at Crystal Red shrimp failed completely — three batches of shrimp, all dead within a month, no obvious cause. The pH was right, the temperature was right, the ammonia and nitrite were zero. I was feeding expensive shrimp food and using RO water because the care sheet said "soft water." What I had not done was test GH. My RO water remineralization was insufficient; the GH was sitting at 2 dGH, far too low for shrimp to molt successfully. They were dying trapped in their old shells. Two minutes with a GH test kit would have caught it. Two minutes I did not take because I did not understand what GH actually measured.

This is the deep-dive companion to the water parameters overview. It covers what GH measures, why it matters for fish and shrimp and plants in different ways, how to test it, how to raise and lower it, the species-specific targets worth knowing, and the often-confused relationship between GH and KH (which is covered in its own deep-dive). The TDS guide covers the broader "everything dissolved in the water" picture, of which GH is one slice.

What Is GH, Exactly?

General Hardness (GH) measures the concentration of dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions in water. These are the two minerals that water hardness tests pick up, and they are the two minerals that matter most for fish and invertebrate biology. Other dissolved minerals — sodium, potassium, iron, trace elements — are not counted in GH even though they contribute to TDS. GH is specifically calcium plus magnesium, nothing else.

GH is measured in two units: degrees of general hardness (dGH), where 1 dGH equals 10 milligrams of calcium oxide per liter, and parts per million (ppm) of calcium carbonate equivalent. The conversion is 1 dGH = 17.8 ppm. Most aquarium test kits report in dGH because the degree scale gives smaller, friendlier numbers (a "4 to 8 dGH" range reads easier than a "70 to 140 ppm" range). The two units refer to the same thing; multiply dGH by 17.8 to get ppm, divide ppm by 17.8 to get dGH.

Water is classified by GH into four broad ranges. Very soft (0 to 4 dGH, 0 to 70 ppm) is what comes out of an RO filter or falls as rain in areas with granitic bedrock. Soft (4 to 8 dGH, 70 to 140 ppm) is typical of the Amazon basin, the Southeast US, and parts of the Pacific Northwest. Hard (8 to 18 dGH, 140 to 320 ppm) is typical of the American Midwest, most of England, and areas with limestone bedrock. Very hard (18+ dGH, 320+ ppm) is what comes out of wells in limestone areas, Lake Malawi, and Lake Tanganyika. Most tap water in North America falls in the 5 to 15 dGH range.

Why GH Matters

Three groups in your tank depend on GH directly: fish, invertebrates, and plants. Each uses calcium and magnesium for different things, and each has different tolerance ranges. The reason GH is "intermediate" difficulty rather than beginner is that you can keep a hardy community tank for years without ever testing GH and never notice — fish and plants get enough calcium and magnesium from almost any tap water. It is when you start keeping shrimp, breeding sensitive species, or running planted tanks with CO₂ injection that GH becomes essential to test.

Fish osmoregulation. Freshwater fish constantly lose salts to their environment (because their bodies are saltier than the surrounding water) and constantly absorb water through their gills. Their kidneys and chloride cells work to maintain internal salt and water balance against this gradient. Calcium and magnesium in the water are part of that balance — calcium ions are used in cell membrane function and muscle contraction, magnesium is used in enzyme reactions. Hard-water fish (livebearers, African cichlids, goldfish) have evolved to extract these minerals from hard water and struggle in very soft water. Soft-water fish (tetras, discus, angelfish) have evolved to conserve minerals and struggle in very hard water. A mismatch between fish and GH does not kill immediately — it stresses the fish over months, leading to shortened lifespans and reduced disease resistance.

Shrimp molting. Shrimp build their exoskeletons from calcium carbonate pulled from the water. Every time a shrimp molts, it sheds its entire shell and grows a new one over the next 12 to 24 hours. The new shell requires a massive, sudden supply of calcium, which the shrimp extracts from the water through its gills. If GH is too low (below 4 dGH), the new shell is too thin and the shrimp dies during or just after the molt. If GH is too high (above 15 dGH), the new shell is too thick and the shrimp cannot break out of the old shell, dying trapped. Shrimp also need the right ratio of calcium to magnesium (roughly 3:1 to 4:1 by ions) — pure calcium without magnesium produces shells that are brittle rather than flexible. This is why you cannot just toss calcium pills into a shrimp tank and call it done; the minerals have to be in the right proportion.

Plant nutrient availability. Plants use magnesium as the central atom in chlorophyll — without magnesium, plants cannot photosynthesize. Calcium is used in cell wall structure and as a signaling ion. Most tap water has enough of both for plant growth, but in very soft water (below 3 dGH) or in RO-remineralized water with the wrong mineral blend, plants develop deficiency symptoms: magnesium deficiency shows as yellowing between the veins of older leaves, calcium deficiency shows as deformed new growth with twisted or stunted leaves. Planted tanks with CO₂ injection sometimes need supplemental magnesium (Epsom salt in tiny doses, 1/4 tsp per 20 gallons) because the plants pull magnesium faster than water changes can replace it.

How to Test GH

Two main options. The API GH & KH Test Kit is a liquid titration test that costs about $10 and runs about 50 tests. You add tank water to a vial, then add GH reagent one drop at a time, swirling between drops, until the color changes from orange to green. The number of drops equals the dGH. The test is accurate but slow — a hard water tank takes 15+ drops and a couple of minutes to test.

The faster option is a TDS meter, which measures total dissolved solids in ppm. TDS correlates with GH but is not the same — TDS includes sodium, potassium, chloride, and everything else dissolved in the water, while GH is only calcium and magnesium. As a rough rule, GH in ppm is about 60 to 70% of TDS in typical tap water (because calcium and magnesium are the bulk of dissolved solids in most municipal water). If your TDS reads 200 ppm, your GH is probably around 120 to 140 ppm (7 to 8 dGH). This is approximate but close enough for routine monitoring. For accurate GH, use the liquid test; for daily monitoring, the TDS meter is faster.

Test GH when setting up a new tank, when adding shrimp, when fish or shrimp are not thriving for unexplained reasons, and after any major water source change (moving, switching from tap to RO, etc.). In a stable tank, monthly GH testing is sufficient — GH does not swing the way ammonia or pH do, because the minerals do not evaporate and are not consumed quickly. The exception is a heavily planted tank, where plants can pull enough calcium and magnesium over months to noticeably lower GH.

How to Adjust GH

To raise GH, the cleanest method is Seachem Equilibrium, a blend of calcium, magnesium, and potassium salts designed to raise GH without affecting KH or pH. Dose 1 tablespoon per 20 gallons to raise GH by about 3 dGH. Dissolve the powder in a cup of tank water first (it dissolves slowly and clouds if added directly), then pour the solution into the tank over a few minutes. Equilibrium is the standard for shrimp keepers remineralizing RO water — it produces the right Ca:Mg ratio and does not spike pH. Salty Shrimp GH+ is the shrimp-keeper equivalent, formulated specifically for Caridina and Neocaridina.

The alternative is crushed coral or aragonite in the filter. These calcium carbonate materials dissolve slowly, raising both GH and KH simultaneously. This is appropriate for hard-water fish (livebearers, African cichlids, goldfish) where you want both minerals up and you want the pH buffering that KH provides. It is not appropriate for shrimp-only tanks where you want GH raised without KH changing — Equilibrium or Salty Shrimp is the right tool for that job. Avoid using pure calcium chloride or pure Epsom salt (magnesium sulfate) — they raise only one mineral and throw the Ca:Mg ratio off, which causes problems for both plants and shrimp over time.

To lower GH, the only effective method is dilution with RO (reverse osmosis) water. RO water has zero GH, zero KH, and zero TDS — it is pure H₂O. Mixing your tap water with RO water in the right proportion lowers all three parameters proportionally. Tap water at 12 dGH mixed 50/50 with RO produces water at 6 dGH. Mixed 75/25 RO to tap, you get 3 dGH. There is no chemical additive that lowers GH without also lowering KH and TDS — the minerals have to be physically removed, and RO is how that happens. A small under-sink RO unit costs $80 to $150 and pays for itself within a year if you keep shrimp or soft-water fish.

Do not try to lower GH by boiling water (the old "boil and cool" trick). Boiling precipitates temporary hardness (calcium carbonate) but does not affect permanent hardness (calcium sulfate, magnesium chloride), and most tap water has enough permanent hardness that boiling changes GH only slightly. Boiling also does not affect KH or TDS meaningfully. It is an old wives' tale from the days before RO was affordable. Use RO; it actually works.

GH Targets for Common Species

Species targets are wider than you might expect for captive-bred fish, because most aquarium fish have been bred in tap water for generations. The "ideal" GH on a care sheet is usually the wild-caught value; the captive-bred tolerance is much wider. Use these as a starting point, not a hard limit.

Snails need high GH for shell formation — low GH causes shell pitting and dissolution over time. If you keep Nerite or Mystery snails and their shells are eroding, test GH; it is almost certainly below 6 dGH. The fix is crushed coral in the filter or Equilibrium dosing, raised gradually over a week. Snails are the canary in the coal mine for low GH — if they are thriving, your GH is probably fine for fish.

GH's Relationship to KH

GH and KH are often confused because they are both "hardness" and both measured in degrees, but they measure completely different things. GH measures calcium and magnesium (minerals fish and shrimp use). KH measures carbonate and bicarbonate (the buffer that keeps pH stable). The two are independent — you can have any combination of high or low GH with high or low KH, and each combination has a name in water chemistry. Hard and alkaline (high GH, high KH) is typical of limestone regions and is what African cichlid keepers want. Soft and acidic (low GH, low KH) is typical of the Amazon and is what discus keepers want. Hard but soft-buffered (high GH, low KH) is unusual but happens with Epsom salt dosing; the GH is high but the pH can still crash because KH is low. Soft but well-buffered (low GH, high KH) is what you get when you use baking soda to raise KH in soft water; pH is stable but fish that need calcium struggle.

The reason this matters is that GH and KH adjustment methods are different. Crushed coral raises both GH and KH. Baking soda raises only KH. Equilibrium raises only GH. RO water lowers both. If you have hard water with low KH (rare but possible), adding baking soda raises KH without changing GH, which stabilizes pH without making the water harder. If you have soft water with low KH (common in the Amazon basin mimicry tank), you may want to raise KH without raising GH — baking soda again. If you have hard water with high KH and want softer water for tetras, only RO dilution works. Knowing which parameter you need to adjust is half the battle.

The full chemistry of KH and pH buffering is covered in the KH deep-dive. The short version: if you only test GH and ignore KH, you will eventually have a pH crash you did not see coming. Test both together, monthly, in any tank you care about.

Frequently Asked Questions