GH and KH

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GH and KH are the two hardness numbers on most aquarium test kits, and they are easy to mix up. GH is general hardness, mostly calcium and magnesium. KH is carbonate hardness, the part of the water that buffers pH. This guide explains the difference using an extension publication we opened, then compares seven test products on Amazon that measure one or both: liquid kits, strips and a digital reader. The honest limit up front: none of the sources we opened gives a GH or KH target for bettas, so this guide explains what the numbers are and how to measure them, and does not tell you what to aim for. Listings were read between 2026-10-13 and 2026-10-17; sources were opened on 2026-10-18.

Quick answer

GH tells you how much hardness minerals are dissolved in the water. KH tells you how well the water resists pH swings. If you want both from one product, the API GH & KH Test Kit lists both. If you also want pH, nitrite and nitrate on the same test, the API 5-in-1 strips list GH and KH among five readings. If you need only one number, there are single-test options from API, sera and Söll.

GH and KH, defined from an extension source

The Southern Regional Aquaculture Center publication Liming Ponds for Aquaculture (SRAC 4100), by Wurts and Masser (revised August 2013), is written for ponds, not tanks. It still gives a clear definition of the two ideas that aquarium kits call GH and KH:

• Hardness is the overall concentration of divalent salts (calcium, magnesium, iron, etc.). Calcium and magnesium are the most common sources. This is what a GH test is aimed at.
• Total alkalinity is the quantity of titratable bases, mainly bicarbonates and carbonates. It buffers pH. This is the idea behind a KH test, although aquarium makers use “KH” and “alkalinity” in different places (the API KH listing says “carbonate hardness”; the Tetra strip listing says “alkalinity”).
• The publication notes the two are often confused, because both are reported in the same unit, mg/L as CaCO3 (the same as ppm).

It also says that when total alkalinity is below 20 mg/L, pH can swing widely, which stresses aquatic animals. The UF/IFAS fact sheet Ammonia in Aquatic Systems (FA031), opened 2026-10-18, adds that the bacteria that clear ammonia stop working effectively below 20 mg/L total alkalinity. Both are written for aquaculture, not betta tanks, so treat 20 mg/L as a reference point rather than a betta rule.

What these sources do not tell you

SRAC 4100 gives a desirable calcium hardness range of 75 to 250 mg/L with a minimum of 20 mg/L for aquaculture animals, and a preferred alkalinity of 50 mg/L or higher for many animals. Those are pond figures. The UF/IFAS betta fact sheet FA212 and the University of Illinois column Bettas Need More Than Bowls do not give a betta GH or KH target. The API GH & KH listing only says hardness should match the fish’s natural habitat. If your water seems to be causing a problem, ask an experienced fish keeper or an aquatic vet.

The seven tests compared

Product	Measures	Format	Range or count on the listing
API GH & KH Test Kit	GH and KH	Liquid drops, 2 tubes	Units not stated in the bullets
API Carbonate Hardness (KH) Test Kit	KH only	Liquid drops, color card	Freshwater and saltwater
sera kH-Test	KH only	Liquid, 15 ml	Fresh and salt water
Söll GH-Test	GH only	Droplet test	0.5 to 21 dH, step 0.5 dH
API 5-in-1 Test Strips	GH, KH, pH, nitrite, nitrate	Strips	100 strips
Tetra EasyStrips 6-in-1	Water hardness, alkalinity, pH, nitrate, nitrite, chlorine	Strips	100 strips, 60 seconds
Pawfly 6 in 1 Digital Kit	GH, KH, pH, nitrate, nitrite, chlorine	Strips plus digital reader	GH 0 to 425 ppm, KH 40 to 180 ppm, 50 strips

The seven picks

Worked example: why the range matters

For example, SRAC 4100 and FA031 both single out total alkalinity below 20 mg/L. The Pawfly reader’s KH range is 40 to 180 ppm, and the listing says a result below range shows the minimum, so it would display 40 ppm for water at, say, 15 ppm. If the listing’s ppm is the same as mg/L as CaCO3 (the listing does not say), that reader could not show you that the water is under 20. The Söll GH-Test, by contrast, reports in dH from 0.5 to 21 in steps of 0.5, so its numbers cannot be compared directly with Pawfly’s ppm readings unless you convert using the maker’s chart. In short, check the unit and the lowest number on a test before you buy it.

How to choose

• Both numbers, one kit: API GH & KH.
• Many readings, low effort: API 5-in-1 or Tetra 6-in-1 strips.
• KH only: API KH or sera.
• GH with a stated range: Söll.
• Digital readout: Pawfly, with the range caveat above.

Related guides

For broader testing, read Best API Freshwater Master Test Kit Chart, Best Aquarium Water pH Test Kit, and Best Freshwater Fish Tank Water Test Kit.

FAQ

What is the difference between GH and KH?

GH measures the minerals that make water hard. KH measures the carbonate buffer that holds pH steady. SRAC 4100 says the two ideas are often confused.

What GH and KH do bettas need?

The sources we opened give no betta target. The API GH & KH listing says hardness should match the fish’s natural habitat. UF/IFAS FA212 gives wild habitat pH but no hardness figures.

Why does KH matter for pH?

SRAC 4100 says low alkalinity, below 20 mg/L, can produce large daily pH swings in ponds. The API KH listing says KH stabilizes aquarium pH.

Are ppm and dH the same?

No. The Söll test reports in dH and the Pawfly reader in ppm, so use the maker’s conversion chart before comparing.

How often should I test?

The API listings advise weekly monitoring and testing when problems appear. The sources we opened give no betta schedule for GH and KH.