What is GH in a fish tank?

GH stands for General Hardness — it measures the concentration of dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions in your water. These two minerals are the primary components of what is commonly called 'hard water.' GH is measured in degrees of hardness: dGH (German degrees of hardness) or ppm (parts per million), where 1 dGH = 17.9 ppm. Soft water has a GH below 4 dGH; very hard water is above 20 dGH. GH matters for fish because calcium and magnesium are essential minerals for biological processes — muscle function, bone/scale development, osmoregulation (the process by which fish manage the movement of water and salts across their membranes). Fish evolved in water with specific GH ranges, and keeping them significantly outside that range causes chronic stress and reduced lifespan.

What is KH in a fish tank?

KH stands for Carbonate Hardness — it measures the concentration of carbonate (CO₃²⁻) and bicarbonate (HCO₃⁻) ions in your water. Despite sharing the word 'hardness' with GH, KH is a fundamentally different measurement. KH is primarily important because it is the pH buffering capacity of your water — carbonates and bicarbonates act as a chemical buffer that neutralises both acids and bases, keeping pH stable. A tank with adequate KH (above 3–4 dKH) will maintain a stable pH even as biological processes produce acids (respiration, nitrification). A tank with near-zero KH has no buffering capacity — pH becomes unstable and can crash rapidly, particularly at night when plants consume oxygen and produce CO₂.

What is the difference between GH and KH?

GH and KH measure different things and have different implications. GH measures calcium and magnesium (the minerals we think of as 'hard water'). KH measures carbonates and bicarbonates (the buffering capacity that controls pH stability). A water source can have high GH and low KH, or low GH and high KH — they do not always correlate. For example, RO water that has been remineralised with calcium and magnesium chloride has high GH but near-zero KH. Rainwater has near-zero GH and near-zero KH. Lake Malawi water has both very high GH and very high KH. The practical implication: GH primarily affects the fish's physiology and osmoregulation. KH primarily affects pH stability. Both matter — but for different reasons.

What should GH and KH be in a freshwater aquarium?

The ideal GH and KH depend entirely on the fish species you are keeping. General community fish (most tetras, rasboras, guppies, corydoras, common livebearers): GH 8–15 dGH, KH 4–8 dKH, pH 6.8–7.5. Soft-water South American fish (German Blue Rams, discus, apistogramma, cardinal tetras, altum angelfish): GH 3–8 dGH, KH 2–5 dKH, pH 6.0–7.0. Hard-water African cichlids (Lake Malawi, Tanganyika, Victoria): GH 15–25 dGH, KH 10–20 dKH, pH 7.8–8.5. Shrimp (neocaridina cherry shrimp): GH 7–15 dGH, KH 2–8 dKH, pH 7.0–7.5. Shrimp (caridina crystal shrimp): GH 4–6 dGH, KH 0–2 dKH, pH 5.8–6.8.

How do I raise KH in my aquarium?

The most effective ways to raise KH in a freshwater aquarium are: Sodium bicarbonate (baking soda) — 1 teaspoon per 50 litres raises KH by approximately 4 dKH. Add dissolved in tank water, never directly. Crushed coral or aragonite in the filter — releases carbonates slowly as water passes through; a natural, gradual method that self-regulates (stops raising KH when equilibrium is reached). Commercial KH buffer products — convenient and pre-measured. Limestone or coral rock as hardscape — adds carbonates gradually but less controllably than filter media. Do not raise KH by more than 2–3 dKH per day to avoid pH shock to fish.

How do I lower GH in my aquarium?

Lowering GH requires diluting the mineral content of your tank water. The main methods are: RO (reverse osmosis) water — pure water with near-zero GH. Blend with tap water to achieve target GH. A 50% RO/50% tap blend halves your tap water's GH. Rainwater — similar properties to RO water (near-zero GH and KH), free but variable in quality. Requires treatment before use. Peat in the filter — releases tannins and humic acids that softens water gradually, also lowers pH. Ion exchange resins — some filter media exchange calcium and magnesium ions for sodium, reducing GH. Note that simple chemical treatments cannot reduce GH — the minerals must be physically diluted or exchanged.

GH and KH in Aquariums — What They Mean, Why They Matter, and How to Fix Them

Quick answer

GH (General Hardness): Measures dissolved calcium (Ca²⁺) and magnesium (Mg²⁺). This is the “hardness” most people mean when they say “hard water.” Affects fish physiology and osmoregulation.
KH (Carbonate Hardness): Measures carbonates and bicarbonates. This is the pH buffer — KH is what stops your pH from crashing. High KH = stable pH. Low KH = pH swings.
Key difference: GH and KH are independent. You can have high GH and low KH, or high KH and low GH. They don’t always move together.
Units: Both measured in dGH/dKH (degrees of hardness) or ppm. 1 dGH/dKH = 17.9 ppm.
Why it matters: The wrong GH or KH for your fish species causes chronic osmotic stress. A collapsing KH causes pH crashes that can kill fish overnight.

GH — General Hardness

What GH actually measures

GH measures the concentration of divalent metal cations in your water — in practice, this almost entirely means calcium ions (Ca²⁺) and magnesium ions (Mg²⁺). These are the minerals that originate in rock — as water flows over and through limestone, chalk, gypsum, and other mineral-rich geology, it dissolves and picks up calcium and magnesium. Water from areas with soft, mineral-poor geology (granite, moorland) has low GH. Water from chalk and limestone areas has high GH.

This is why UK water hardness varies so dramatically by region: soft in Scotland, Wales, the Peak District, and the Lake District (water from granite or moorland catchments); very hard in London, East Anglia, and the South East (water from chalk aquifers). London tap water typically tests at 20–25 dGH — very hard.

17.9 ppm

= 1 dGH (one degree of General Hardness)

Most test kits report in either dGH or ppm (mg/L). Your water company’s hardness report is usually in mg/L as CaCO₃ — divide by 17.9 to convert to dGH. A water report saying “357 mg/L CaCO₃” means approximately 20 dGH — very hard.

Why GH matters for fish

Fish are osmoregulators — they constantly manage the movement of water and dissolved ions across their body membranes to maintain the right internal salt balance. The difficulty of this process depends on how different the surrounding water is from the fish’s body fluids:

In very soft water (low GH): Freshwater fish face an osmotic challenge where water constantly enters their body (by osmosis) and dissolved minerals are lost to the water. Their kidneys work hard to expel excess water and conserve minerals. This is why hard-water fish (cichlids, livebearers) struggle in soft water — they are adapted to environments where this challenge is less severe.
In very hard water (high GH): The reverse challenge. Soft-water fish (discus, German Blue Rams, most South American tetras) struggle in hard water because their osmoregulatory systems are adapted to very low mineral environments. Chronic exposure to high GH causes osmotic stress, immune suppression, and shortened lifespan in soft-water species.
For invertebrates: Calcium is critical for exoskeleton construction in shrimp, snails, and other invertebrates. Very soft water (GH below 4 dGH) causes moulting problems in shrimp and thin, fragile shells in snails.

GH targets by fish type

Soft-water fish (German Blue Ram, discus, cardinal tetra, apistogramma, rummynose tetra): 3–8 dGH
General community fish (most tetras, rasboras, danios, corydoras, common plecos): 8–15 dGH
Hard-water livebearers (guppies, mollies, platies, swordtails): 12–20 dGH
African cichlids (Lake Malawi, Tanganyika): 15–25+ dGH
Goldfish: 8–15 dGH
Neocaridina shrimp (cherry shrimp): 7–15 dGH
Caridina shrimp (crystal red, bee shrimp): 4–6 dGH

KH — Carbonate Hardness

What KH actually measures

KH measures the concentration of carbonate (CO₃²⁻) and bicarbonate (HCO₃⁻) ions in your water. In most freshwater aquariums, it is primarily bicarbonate that dominates (carbonate is only significant at very high pH). Bicarbonate in water comes from dissolved CO₂ reacting with carbonates in rock — in limestone-rich areas, both KH and GH are typically high, which is why they often move together. But they are still independent measurements.

KH’s critical role: pH buffering

This is the single most important thing to understand about KH:

KH is your pH safety net. Biological processes in an aquarium constantly produce acids — nitrification (the bacterial conversion of ammonia to nitrite to nitrate) produces nitric acid; respiration by fish and plants releases CO₂ which forms carbonic acid; decomposing organic matter produces organic acids. Without anything to neutralise these acids, pH would steadily drop in any mature tank.

Bicarbonate ions in the water neutralise these acids through a buffer reaction — essentially absorbing the acid before it can change the pH. As long as KH remains above approximately 3–4 dKH, this buffering process can compensate for normal aquarium acid production and pH stays stable.

Low KH = pH crash risk

A tank with KH below 2–3 dKH has very little buffering capacity. In a planted tank or any tank with significant biological activity, pH can crash from 7.0 to 5.5–6.0 overnight as CO₂ accumulates (from respiration at night when plants aren’t photosynthesising) with nothing to buffer it. pH crashes kill fish — the change is often too rapid for fish to adapt, and low pH impairs gill function. If you ever find fish dead or gasping in the morning with no obvious cause, and your KH is very low, a nighttime pH crash is a strong candidate.

The CO₂ / KH / pH relationship

In planted tanks with CO₂ injection, understanding this triangle matters:

CO₂ dissolves in water forming carbonic acid, which lowers pH.
KH buffers against this — higher KH means you need more CO₂ to achieve the same pH reduction.
At any given KH, there is a predictable relationship between CO₂ concentration and pH (the KH/pH/CO₂ table used by planted tank keepers). This is why CO₂ injection typically requires a meaningful KH to be safely controllable.
In very low KH water with CO₂ injection, pH becomes unpredictable and hard to control — a small change in CO₂ causes a large pH swing.

Common confusion

“My pH is stable, so my KH must be fine.”

Reality

pH can appear stable during the day in a planted tank because plant photosynthesis consumes CO₂ and raises pH — temporarily masking a low-KH stability problem. At night, when photosynthesis stops but respiration continues, CO₂ accumulates, carbonic acid builds up, and with no KH to buffer it, pH crashes. The only way to know if your KH is adequate is to test it directly, not to infer it from pH readings.

KH targets for freshwater tanks

General community tanks (most fish species): 4–8 dKH — adequate buffering, stable pH.
Soft-water species tanks (GBR, discus, apistogramma): 2–5 dKH — lower KH to allow the lower, slightly acidic pH these fish prefer, but enough for minimal buffering.
African cichlids: 10–20 dKH — high alkalinity to maintain the hard, alkaline conditions of the Rift Lakes.
CO₂ planted tanks: 3–5 dKH minimum — enough to keep pH manageable under CO₂ injection. Very low KH with CO₂ makes pH control unpredictable.
Caridina shrimp: 0–2 dKH — crystal shrimp require very low KH and correspondingly low pH. This makes these tanks vulnerable to pH swings and requires more frequent monitoring.

How to test GH and KH

Both GH and KH are tested using drop-based titration test kits. You add drops of reagent to a measured water sample until the colour changes — each drop equals a certain number of degrees (usually 1 dGH or 1 dKH per drop, depending on the kit).

API GH & KH Test Kit: The most common and widely available. Accurate, inexpensive. Comes with separate reagents for GH and KH.
Salifert and Sera test kits: More precise colour changes, useful for very soft water where colour transitions can be subtle.
Digital TDS meters: Measure total dissolved solids — correlates with GH but does not distinguish GH from KH or other dissolved substances. Not a substitute for dedicated GH/KH tests.
Test strips: Available for GH/KH but less precise than drop kits. Useful for a quick check, not for precision parameter management.

How to raise and lower GH and KH

Raising GH

Seachem Equilibrium / commercial GH+ products: Designed specifically to raise GH without significantly affecting KH or pH. Contains calcium and magnesium in appropriate ratios.
Calcium chloride + magnesium sulphate (Epsom salt): For more experienced keepers remineralising RO water. CaCl₂ raises GH; MgSO₄ adds magnesium. Used together in appropriate ratios they replicate the Ca:Mg balance of natural water.
Adding crushed coral or limestone: Raises both GH and KH — not suitable if you only want to raise GH.
Switching to harder tap water blend: If your tap water is already hard, blending it with RO water in appropriate ratios achieves any target GH.

Lowering GH

RO (reverse osmosis) water: The most reliable method. Blend with tap water in the desired ratio to achieve target GH. A 50:50 blend halves your tap water GH.
Rainwater: Near-zero GH and KH. Requires treatment before use. Effective for reaching soft-water targets when blended with tap water.
Ion exchange resins (sodium cycle): Exchange calcium and magnesium ions for sodium. Reduces GH but increases sodium content — appropriate for some applications, less so for very sensitive species.

Raising KH

Sodium bicarbonate (baking soda): The most cost-effective method. 1 level teaspoon (approximately 6g) per 50L raises KH by around 4 dKH without significantly affecting GH. Dissolve fully in tank water before adding. Never add directly as solid — it reacts with the water releasing CO₂ and can cause a brief pH spike.
Crushed coral in the filter: Releases carbonates slowly and self-regulates — as KH rises, the reaction slows. A natural, gentle method that won’t spike KH. Takes 24–72 hours to begin raising KH noticeably.
Commercial KH buffer products: (Seachem Alkalinity Buffer, API proper pH products). Pre-measured and convenient. Follow manufacturer’s dosing guidelines carefully.
Limestone or coral rock as hardscape: Raises KH (and GH) very gradually. Over months, can meaningfully raise a tank’s baseline KH.

Lowering KH

RO or rainwater for water changes: Dilutes KH with each water change. Consistent use gradually lowers the baseline KH of a tank.
Peat filtration: Releases tannins and humic acids that consume carbonates and lower KH (and pH). Gradual — useful for soft-water species tanks. Also stains water brown (which can be desirable for some biotopes).
CO₂ injection: In planted tanks, CO₂ reacts with bicarbonates — but this is a daily cycling effect, not a permanent reduction. When CO₂ is off, KH returns.
Peat moss in a mesh bag in the filter: A convenient peat delivery method without using peat substrate.

Common mistake

Trying to lower pH directly rather than adjusting KH. Many fishkeepers trying to achieve low pH for soft-water fish add pH-down products (typically phosphoric acid) — these products directly acidify the water without addressing KH. The result is a low pH with the same underlying KH, meaning the pH is now both lower and less stable than before — it will bounce unpredictably as the tank’s KH struggles to buffer against both natural acid production and the added acid. The correct approach for soft-water fish is to reduce GH and KH using RO or rainwater blending, then allow the pH to reach its natural equilibrium at the lower KH — which will be lower and more appropriate for soft-water species without the instability of pH-down products.

The relationship between GH, KH and pH

These three parameters are interrelated but not directly linked in a simple way:

High KH tends to push pH higher because carbonate ions consume hydrogen ions (H⁺), reducing acidity. This is why African cichlid tanks — which require very high KH — also have very high pH (8.0+). You cannot easily maintain high KH at low pH.
High GH does not directly affect pH. Hard water can be acidic or alkaline depending on KH. London tap water is both very hard (high GH) and slightly alkaline (pH 7.5–8.0) primarily because of its very high KH.
Very low KH allows pH to be determined primarily by biological processes — CO₂, organic acids, plant activity. This is why soft-water tanks with low KH can have pH in the 6.0–6.5 range naturally, without any pH adjustment, once CO₂ and organic acids have pulled pH down.

3 dKH

The minimum KH for safe pH stability in most aquariums

Below 3 dKH, the buffering capacity of your water becomes insufficient to compensate for normal acid production in a cycled tank. pH becomes vulnerable to overnight crashes, particularly in tanks with heavy biological activity, CO₂ injection, or dense plant life. Test your KH — if it’s below 3, raising it should be a priority before it becomes a fish emergency.

GH and KH checklist — before you stock a new tank

Test your tap water’s GH and KH — your water company’s report is a starting point but actual readings vary. Test directly.
Look up the GH and KH requirements for the specific species you plan to keep — not just general community guidance.
If your tap water GH or KH is too high for your target species, plan your RO or rainwater blending ratio before stocking.
Ensure your KH is at least 3 dKH for general tanks before stocking — even if your target fish prefer softer water, some buffering is essential.
If you’re using RO water, always remineralise before use — 100% RO water has near-zero GH and KH, which is too soft and unstable for virtually all fish except specialist caridina shrimp setups.
Check your parameters again after 24–48 hours of adding any substrate, hardscape, or filter media — these can change GH and KH.

App-aquatic tracks your GH, KH, and pH over time — scan your test strips with your camera and build a complete water history so you can see if your parameters are drifting before it becomes a problem.

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Frequently asked questions

Can I have high GH and low KH at the same time?

Yes — GH and KH are independent. RO water remineralised with calcium chloride and magnesium sulphate can have high GH (adding calcium and magnesium) but near-zero KH (no carbonates added). Conversely, water from certain mineral springs may have significant bicarbonate content (high KH) but low calcium and magnesium (low GH). Most tap water in chalk or limestone areas has both high GH and high KH because the same geology that contributes calcium also contributes carbonates — but this co-occurrence is coincidental, not inevitable.

How often should I test GH and KH?

For a stable, established tank using consistent tap water, testing GH and KH monthly is usually sufficient. If you are blending RO or rainwater with tap water, test every water change to confirm your blend is achieving the target parameters — small variations in your blend ratio add up over time. If you have a tank with very low KH (soft-water species, caridina shrimp), test KH weekly — low-KH tanks are vulnerable to parameter drift from CO₂ fluctuations, bioload changes, and evaporation top-up accumulation.

My fish seem fine but my GH is wrong for the species — should I change it?

Fish are adaptable in the short term — a GBR in London tap water (GH 20+ dGH) may appear outwardly fine for months while experiencing chronic internal stress. The effects of wrong GH manifest over time: reduced lifespan, greater susceptibility to disease, failure to display full colour, reduced reproductive success. Gradual adjustment toward appropriate GH is recommended. Avoid sudden large changes — shift parameters over weeks by blending increasing proportions of RO water into your water changes. A 10% shift in GH per water change is a safe rate of change.

Is dGH the same as ppm?

No — they are different units for the same measurement. 1 dGH = 17.9 ppm (mg/L as CaCO₃). To convert dGH to ppm: multiply by 17.9. To convert ppm to dGH: divide by 17.9. Your water company’s hardness figures are almost always given in mg/L as CaCO₃ (equivalent to ppm). Divide that number by 17.9 to get dGH. For example, “hardness 357 mg/L CaCO₃” = 357 ÷ 17.9 = approximately 20 dGH — which is very hard water.

What do GH and KH mean — and why do they matter in your aquarium?