Water hardness: how to adjust GH and KH
Water hardness is one of the most under-discussed parameters in the freshwater aquarium and pond hobby, and one of the most consequential. The fish that die mysteriously within 24 hours of a water change, the plants that fail to thrive despite appropriate lighting and nutrients, the snails that cannot build healthy shells, the shrimp that fail to molt — in a meaningful percentage of these cases, the underlying problem is a GH or KH that is wrong for the species, and the fix is the kind of parameter adjustment that this article is going to walk you through. The adjustment is not difficult, but it is also not forgiving, and the keeper who does the adjustment without understanding the chemistry is the keeper who produces a fish-kill.
This article is going to walk you through what GH and KH are (and what they are not), why they matter, how to raise or lower each, the species-specific targets for the common freshwater fish and invertebrates, and the four mistakes that turn a parameter adjustment into a disaster. I am a pond and aquarium keeper with 20+ years of experience ranging from soft-water Amazon biotopes to hard-water African cichlid tanks, and the water chemistry is the foundation of every successful freshwater system I have ever run.
The article is focused on freshwater. The saltwater parameters (calcium, alkalinity, magnesium) are the reef trinity I covered in an earlier article, and the adjustment chemistry is different.
What GH and KH are, and what they are not
GH (General Hardness). The measure of the total concentration of calcium (Ca²⁺) and magnesium (Mg²⁺) ions in the water, expressed as the equivalent concentration of calcium carbonate (CaCO₃) in parts per million (ppm) or in degrees German hardness (dGH, where 1 dGH = 17.86 ppm CaCO₃). The standard unit in the aquarium hobby is ppm or dGH, and the conversion is 1 dGH = 17.86 ppm.
GH is a measure of the "permanent hardness" — the hardness that cannot be removed by boiling (because the calcium and magnesium ions are stable in solution at high temperature). The hardness is "general" because it includes all the divalent cations, not just calcium and magnesium (although calcium and magnesium are the dominant contributors in most natural waters).
KH (Carbonate Hardness). The measure of the concentration of carbonate (CO₃²⁻) and bicarbonate (HCO₃⁻) ions in the water, also expressed as the equivalent concentration of calcium carbonate in ppm or in degrees German hardness (dKH, where 1 dKH = 17.86 ppm CaCO₃). KH is the measure of the water's buffering capacity — its ability to resist a change in pH when an acid is added.
KH is a measure of the "temporary hardness" — the hardness that can be removed by boiling (because the bicarbonate is converted to carbonate, which precipitates as calcium carbonate when boiled). The hardness is "carbonate" because it is specifically the carbonate and bicarbonate ions that contribute.
The difference. GH and KH are different things, and they can be independently high or low. A water can have a high GH and a low KH (hard, poorly buffered — typical of well water in many regions). A water can have a low GH and a high KH (soft, well buffered — typical of some municipal waters). A water can have a high GH and a high KH (hard, well buffered — typical of African rift lake water). A water can have a low GH and a low KH (soft, poorly buffered — typical of Amazonian water and of pure RO/DI water). The four combinations are biologically distinct, and the right target for a given species depends on the species' natural habitat.
What GH and KH are not. GH is not the same as TDS (total dissolved solids), which is the measure of all dissolved ions (not just calcium and magnesium). KH is not the same as pH, which is the measure of the water's acidity or alkalinity. GH and pH are related (high GH often correlates with high pH), and KH and pH are related (low KH often correlates with low pH), but the correlations are not absolute, and the parameters are measured separately.
Why GH and KH matter
GH and KH matter for three reasons: fish health, plant health, and biological stability.
Fish health. Fish have evolved in water with specific hardness ranges, and their physiology is adapted to those ranges. The osmoregulation (the fish's management of the salt and water balance across the gills, the skin, and the kidneys) is tuned to a specific GH. A fish kept in water that is significantly harder or softer than its native range is in a state of chronic osmotic stress, which produces weakened immune function, increased susceptibility to disease, and (in some cases) failure to thrive or to breed.
The osmoregulatory stress is most pronounced in species that are adapted to extreme hardness or softness. African cichlids (Lake Malawi, Lake Tanganyika) are adapted to very hard, alkaline water (GH 200–400+ ppm, KH 150–300+ ppm, pH 7.8–8.6) and suffer in soft, acidic water. South American tetras, discus, and wild bettas are adapted to very soft, acidic water (GH 20–80 ppm, KH 0–40 ppm, pH 5.5–6.8) and suffer in hard, alkaline water. Most common community fish (livebearers, danios, barbs, most rasboras) are adaptable to a wider range, but each species has a preferred range, and the keeper who provides the preferred range is the keeper whose fish thrive.
Plant health. Aquatic plants are sensitive to GH and KH, particularly the high-tech plants (the stemmed plants, the carpeting plants, the red plants) that are the focus of the planted tank hobby. The calcium and magnesium in the water are essential nutrients for the plants, and the GH is the source of those nutrients. The KH affects the plants through the buffering of the pH and through the availability of CO2 (which is the carbon source for the plants, and which is more available at lower pH). The planted tank that has a GH of 50–150 ppm and a KH of 30–80 ppm is a planted tank that supports healthy plant growth.
Biological stability. The KH, in particular, is the buffer that prevents the pH from crashing. The pH of a tank with low KH can drop rapidly when the biological activity produces acids (the nitrification process produces hydrogen ions, the respiration of the fish and the bacteria produces carbon dioxide, the decomposition of organic matter produces organic acids). The drop in pH can be sudden, can be severe, and can be lethal to the fish. The KH is the buffer that absorbs the acids and prevents the pH from dropping. The tank with a KH of at least 40 ppm is a tank that resists pH crashes. The tank with a KH below 20 ppm is a tank that is at risk of a pH crash, especially in a heavily-stocked or heavily-planted system.
The species-specific targets
The right target depends on the species. The general framework:
Soft water species (South American tetras, discus, wild bettas, most South American dwarf cichlids, most soft-water plants). GH 20–80 ppm, KH 0–40 ppm, pH 5.5–6.8. The water in the natural habitat is very soft, very low in minerals, and slightly acidic.
Medium water species (most community fish, most livebearers, most danios, most barbs, most rasboras, most gouramis). GH 80–200 ppm, KH 40–120 ppm, pH 6.8–7.6. The water in the natural habitat is moderate in minerals and slightly acidic to slightly alkaline.
Hard water species (African cichlids, livebearers like mollies and guppies and platies, goldfish, koi, most pond fish). GH 200–400+ ppm, KH 120–300+ ppm, pH 7.6–8.6. The water in the natural habitat is very hard, very high in minerals, and alkaline.
Invertebrates (most freshwater shrimp, most snails). GH 100–250 ppm (for the calcium needed for shell and exoskeleton formation), KH 50–150 ppm, pH 6.8–7.6. The shrimp are particularly sensitive to GH — too low a GH produces molting failures, too high a GH produces osmotic stress. The target for most dwarf shrimp is GH 100–150 ppm, for most snails GH 150–250 ppm.
The importance of matching the species to the water. The keeper who is setting up a new tank should choose the species to match the source water, not the other way around. The keeper who has hard well water and wants to keep soft-water tetras is going to spend a lot of time and money adjusting the water, and the result will be a tank that is constantly fighting against the source water. The keeper who matches the species to the water is the keeper who has a stable, healthy tank with minimal intervention.
How to raise GH and KH
The methods for raising GH and KH are different, because the two parameters are different.
Raising GH. The standard method is to add a GH+ product (Seachem Equilibrium, API GH/KH Booster, or a similar calcium/magnesium supplement). The product is dosed according to the label, and the GH is raised to the target. The pure calcium chloride and magnesium chloride salts (sold as "GH booster" or as separate calcium and magnesium chloride) are an alternative, and the dosing is more precise (the keeper can adjust the calcium and magnesium independently). The crushed coral substrate, which slowly dissolves calcium and carbonate into the water, is a passive method that raises both GH and KH, but the rate of dissolution is variable and the effect is slow.
Raising KH. The standard method is to add a KH+ product (Seachem Alkaline Buffer, API KH Booster, sodium bicarbonate, or potassium bicarbonate). Sodium bicarbonate (baking soda) is the cheapest option, and the dose is straightforward: 1 teaspoon per 50 gallons raises the KH by approximately 1 dKH (17.86 ppm). The crushed coral substrate also raises KH (along with GH). The aragonite substrate (a calcium carbonate-based sand) is similar to crushed coral but with a different texture and dissolution rate.
Raising both GH and KH simultaneously. The Rift Lake cichlid salt mix (a pre-mixed combination of calcium carbonate, magnesium carbonate, and trace elements) is the standard for African cichlid keepers, and the mix is dosed to raise both GH and KH to the target. The commercial "African cichlid salt" products (Seachem Cichlid Lake Salt, API Cichlid Salt) are pre-formulated and are the easiest option for the keeper who is setting up an African cichlid tank.
How to lower GH and KH
Lowering GH and KH is more difficult than raising them, because the standard method is to dilute the source water with RO/DI water or rainwater. The dilution is the only practical method for lowering GH and KH; the chemical methods (acid buffers, ion exchange resins) are not recommended for the typical aquarium keeper.
The RO/DI dilution method. A reverse osmosis/deionization unit produces water with GH and KH near zero. The RO/DI water is mixed with the source water in a ratio that produces the target GH and KH. For a source water of GH 250 ppm and a target GH of 80 ppm, the mix is approximately 1 part source water to 2 parts RO/DI water (the exact ratio depends on the source water's GH and the target). The KH is similarly calculated. The mixed water is tested with a GH test kit and a KH test kit, and the mix is adjusted until the target is hit.
The rainwater method. Rainwater, in most regions, has very low GH and KH. The rainwater can be used as a diluent in place of RO/DI water, but the rainwater must be tested for contaminants (acid rain, particulates, dissolved organics) and should be filtered before use. The rainwater method is less reliable than the RO/DI method, and the RO/DI method is the standard for the keeper who needs to lower GH and KH consistently.
The peat method. Peat (specifically, peat pellets sold for aquarium use) releases tannins and humic acids into the water, which lower the pH and, to a lesser extent, the KH. The peat method is a slow, soft method for adjusting pH and KH in a soft-water tank, and the method is most appropriate for the South American biotope keeper. The peat does not significantly lower the GH.
The almond leaf and Indian almond leaf method. Similar to the peat method, the almond leaf releases tannins and humic acids into the water, which lower the pH and, to a lesser extent, the KH. The almond leaf is most appropriate as a supplement to the RO/DI dilution method, and the leaf is a popular addition to betta tanks and to soft-water shrimp tanks.
The four mistakes that turn a parameter adjustment into a fish-kill
Mistake 1: Changing the parameters too fast. The single most common mistake. The keeper measures the GH at 250 ppm, decides to lower it to 80 ppm, and adds a large amount of RO/DI water in a single water change. The GH drops from 250 ppm to 80 ppm in a matter of hours. The fish, which were osmoregulated to the 250 ppm water, are suddenly in 80 ppm water. The osmoregulation system is overwhelmed. The fish goes into osmotic shock, and the result is a stressed, sick, or dead fish. The fix is to make the parameter change gradually — no more than a 10–20% change in GH or KH per day, and ideally no more than a 10% change per week. The gradual change allows the fish to adjust its osmoregulation.
Mistake 2: Changing the parameters without testing the source water. The second most common mistake. The keeper buys a GH+ product and adds it to the tank, without testing the source water or the tank water. The GH was already at 200 ppm, the addition raises it to 350 ppm, and the fish (which are soft-water species) are now in a tank that is too hard for them. The fix is to test the source water and the tank water before any adjustment, and to calculate the dose based on the starting value and the target value.
Mistake 3: Using the wrong product for the target parameter. The third most common mistake. The keeper wants to raise the GH and adds a KH+ product, or the keeper wants to raise the KH and adds a GH+ product. The parameters are different, and the products target different parameters. The fix is to read the label of the product carefully, and to verify that the product raises the intended parameter. The Seachem Equilibrium raises GH (calcium and magnesium) but not KH. The Seachem Alkaline Buffer raises KH (bicarbonate) but not GH. The two products are not interchangeable.
Mistake 4: Forgetting to test the parameters after a water change. The fourth most common mistake. The keeper makes a 30% water change with water that has a different GH and KH than the tank, and does not test the parameters after the change. The parameters have shifted, and the fish are in the shifted water. The fix is to test the parameters after every significant water change, and to verify that the change has not produced a problematic shift.
The test kits
The test kits are essential. The right kits are:
GH test kit. A liquid-drop test kit (API, Salifert, Hanna, JBL) is the standard. The kit comes with a titration reagent that is added drop by drop to a water sample, and the number of drops is proportional to the GH. The test is accurate to within 1–2 dGH, which is sufficient for most purposes. API is the most common entry-level option; Salifert and Hanna are more accurate and worth the upgrade for the serious keeper.
KH test kit. A liquid-drop test kit, similar to the GH kit. The same brands (API, Salifert, Hanna) make KH kits. The test is accurate to within 0.5 dKH.
TDS meter (optional but useful). A digital meter that measures the total dissolved solids in the water. The TDS is not a substitute for the GH and KH tests (it measures all dissolved ions, not just calcium, magnesium, and bicarbonate), but it is a useful cross-check. A sudden change in TDS is a signal that something has changed in the water.
The test frequency depends on the system. A stable, well-established tank can be tested monthly. A new tank, a tank undergoing adjustment, or a tank with parameter problems should be tested weekly. A tank with a known KH problem (low KH, frequent pH crashes) should be tested before every significant water change.
The practical workflow
The right workflow for setting up a new tank with a target GH and KH:
- Test the source water. GH, KH, pH. Record the values.
- Determine the target GH, KH, and pH for the species. Match the source water to the species when possible.
- If the source water does not match the target, calculate the adjustment. For raising, calculate the dose of the appropriate product. For lowering, calculate the ratio of source water to RO/DI water.
- Prepare the adjusted water in a mixing container. Mix the source water, the RO/DI water (if needed), and the appropriate products. Stir thoroughly.
- Test the adjusted water. Verify that the GH, KH, and pH are at the target.
- Make the water change with the adjusted water. Change no more than 25–30% of the tank volume per water change.
- Test the tank water after the change. Verify that the change has not produced a problematic shift.
- Repeat weekly, with no more than 10–20% change in the parameters per day, until the target is reached.
The bottom line
GH and KH are foundational parameters in the freshwater aquarium and pond hobby. The right target depends on the species, and the right adjustment is a gradual, tested, calculated process. The keeper who understands the chemistry, who tests the parameters, who adjusts gradually, and who matches the species to the water is the keeper whose fish thrive.
The four mistakes — changing too fast, changing without testing, using the wrong product, and not testing after the change — are the most common ways that parameter adjustments go wrong. The fix in each case is awareness, patience, and discipline. The chemistry is not difficult. The chemistry is a series of small, deliberate steps that any keeper can execute. The right chemistry is the foundation of the healthy tank. The right chemistry is the framework above, applied with rigor.
The fish that thrives in the right water is a fish that is not in osmotic stress, that has a strong immune system, that breeds readily, and that lives a full life. The fish that struggles in the wrong water is a fish that is constantly fighting its environment, and the fish that is fighting its environment is the fish that eventually loses. The keeper's job is to provide the right water. The water is the foundation. The foundation is the framework above.
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About the author
Kenji Tanaka is a pond and aquarium keeper with 20+ years of experience ranging from indoor reef tanks to multi-thousand-gallon koi ponds, and including a wide range of freshwater biotope setups. He has worked with both hobbyists and small commercial aquaculture operations on water chemistry, system design, and long-term husbandry, and writes regularly on the practical realities of freshwater aquarium and pond keeping.
