Aquarium GH and KH Explained

GH, KH and pH answer different questions

Hardness can become confusing because water-company reports, aquarium kits and species descriptions use different terms and units. Start by separating the properties. A sample can have a reassuring pH reading while its mineral profile or buffering differs from what you expected.

GH, general hardness, describes the hardness associated mainly with dissolved calcium and magnesium. It helps characterise the water that fish and other freshwater animals live in. The appropriate range depends on the exact species and the long-term conditions you can maintain.

KH, commonly called carbonate hardness in aquarium keeping, is used by aquarium tests to provide alkalinity/buffering information. Bicarbonate and carbonate are important contributors in many fresh waters. A typical aquarium KH test is not simply a count of calcium in the sample, and the terminology is less precise than the everyday label may suggest.

pH describes acidity or alkalinity at the point of measurement. It is not the same as alkalinity as buffering capacity. Measuring pH does not tell you the GH, and measuring GH does not automatically give the KH.

Keep the units with the number

Aquarium kits may report degrees of general hardness (dGH) or carbonate hardness (dKH). Supplier reports often express hardness in mg/L as calcium carbonate (CaCO₃). A bare number such as “10 hardness” is incomplete until you know the property and unit.

One degree of German hardness corresponds to approximately 17.8 mg/L as CaCO₃, using a rounded conversion; use the conversion table supplied with your kit. For a like-for-like general-hardness comparison, 10 dGH is therefore approximately 178 mg/L as CaCO₃. Conversely, 180 mg/L as CaCO₃ is about 10 dGH. The rounding does not make a colour or drop test precise to several decimal places.

Convert only the relevant hardness measure. A supplier's calcium-only concentration is not automatically total hardness, and a general-hardness result cannot be converted into KH just because both can be expressed using calcium-carbonate equivalents. Use the kit's own conversion table where supplied.

Test the source and the aquarium separately

Establish GH and KH for the water you will use before choosing livestock. Then test the aquarium to see whether the setup maintains similar conditions or changes them. Keep both samples labelled; combining their results in one notebook line makes later comparisons unreliable.

The API GH/KH kit is a focused addition when your existing kit lacks hardness. It does not replace ammonia, nitrite and nitrate tests. Follow the supplied sample volume, drop-by-drop method and endpoint instructions; do not assume every drop kit uses the same procedure.

For a keeper starting without a testing kit, NT Labs Multi-Test includes GH and KH alongside pH and the waste checks. Choose based on the whole kit you need. Its separate nitrogen reporting issue relates to ammonia, nitrite and nitrate, rather than making GH and KH interchangeable.

Why tank hardness and buffering can change

Source water is only the starting point. Some rocks, substrates and additives affect the aquarium's chemistry. Biological processes can consume buffering, while water changes introduce the composition of the replacement water. The effect depends on the actual setup, not just whether a tank is described as “natural”.

Evaporation removes water while leaving dissolved minerals behind. Repeated topping up is therefore a different operation from removing aquarium water and replacing it. If your hardness trend changes, review top-ups, water changes and source composition together rather than assuming one reading proves a faulty kit.

Planted substrates sold to influence pH or hardness need particular attention to the manufacturer's stated behaviour. A buffering substrate can make the relationship between tap KH and tank pH more complicated. Ask for help when planning a managed soft-water setup rather than applying a generic buffer dose.

What is a good GH or KH?

There is no universal ideal pair for all freshwater fish, shrimp, snails and plants. Start with species-specific requirements, then assess whether you can maintain them consistently across water changes. A shop's short-term holding conditions are not automatically a long-term target for your aquarium.

Low KH deserves attention when pH stability is a concern, but it is not a reason to add a buffering product blindly. Deliberately maintained low-alkalinity systems exist and require appropriate methods and expertise. In a general home aquarium, investigate a change in KH together with pH, source water and recent maintenance.

Likewise, hard tap water is not automatically “bad water”. It may suit an appropriate livestock plan. Choosing suitable inhabitants can be more sustainable than designing a preparation process you cannot repeat every week.

Before trying to raise or lower a number

1. Repeat an unexpected reading with the correct test procedure and units.
2. Compare source and aquarium results, including pH and the recent trend.
3. Review substrate, rocks, water treatment, top-ups and water changes.
4. Establish the needs of the exact livestock and the reason for a proposed change.
5. Get a controlled, repeatable preparation plan if modification is genuinely necessary.

A normal dechlorinator is not a general hardness reducer. A household softener is not the same as reverse-osmosis water prepared for fish, because its ion-exchange process changes composition differently. Avoid treating pH-down products as an all-purpose way to create the correct soft-water habitat.

Do not chase a rapidly changing set of targets by adding several products. Where modification is required, use manufacturer instructions and experienced advice, with testing of prepared replacement water before it reaches livestock. Our tap-water guide is the next step for a new setup; the testing routine helps with ongoing records.

If buffering is declining while a new tank is being established, use low KH during fishless cycling. For the separate acidity measurement and suitable test ranges, read aquarium pH explained.

Common questions