KH / Alkalinity refers to the carbonate hardness of water (often expressed in dKH, meq/L, or ppm as calcium carbonate). It represents the water’s buffering capacity — its ability to resist pH swings.

In marine / reef aquariums, maintaining stable alkalinity is critical because it helps control pH, supports coral calcification (alongside calcium and magnesium), and prevents sudden chemistry shifts.

The Salifert kit is designed to measure KH / alkalinity in saltwater / marine systems (though Salifert also offers a freshwater KH / alkalinity version for freshwater aquaria).

Calcium alone cannot form the skeletal material of corals and allow calcareous algae to grow. Some other substances are needed as well. A few other constituents are carbonate and bicarbonate. These two substances also have a major impact on the stabilization of the pH in the proper range of 8.1 – 8.4. Such stabilization is also called buffering.

The total carbonate and bicarbonate concentration is also called carbonate alkalinity or carbonate hardness. The only difference between alkalinity and carbonate hardness is a conversion factor.

NSW has an alkalinity of approx. 2.7 meq/L or approx. 7.5 dKH when expressed as carbonate hardness.

For a stable system the alkalinity or carbonate hardness should have a value similar to NSW or slightly higher and should preferably not fluctuate by more than 5%. This means a maximum fluctuation of 0.14 meq/L or 0.4 dKH.

Therefore an alkalinity test kit should be capable in measuring in steps smaller than 0.14 meq/L.

Conclusion: 

Since the major buffer components used for coral and calcareous algae growth are bicarbonate and carbonate, they should be added to correct any decrease in alkalinity or carbonate hardness.

A proper formulated buffer should function in such a way that the corrective measures results in a long lasting effect and should not upset the pH of the system. The alkalinity or carbonate hardness should be kept as stable as possible requiring highly sensitive and accurate means for testing.

The Salifert KH/Alk test is very straightforward. It measures in sufficient small steps of 0.1 meq/L or 0.3 dKH  with a sharp color change. This makes detection of important yet small change possible.

The kit can perform approx. 100 - 200 measurements.

The Salifert KH + pH Buffer makes correction of the alkalinity or carbonate hardness simple and does not upset the pH of the system. Within 24 hours it will acquire the pH corresponding to other aquarium parameters. Should the pH remain low within 24 hours of corrective measures then an insufficient gaseous exchange (inefficient aeration) is quite often the cause.

A typical Salifert Alkalinity / KH Test Kit includes:

• One or more reagent bottles

• A graduated syringe or dropper for accurate reagent dosing

• A test vial / tube

• Instruction sheet with color chart or endpoint guidelines

• Packaging / box to hold all these items

• Enough reagent to perform about 100 tests (some listings state “100–200 (typically closer to 100)” tests)

Because precise volumes are used, reagent supply is generous relative to each test’s demand.

Principle & Chemistry

1. Sample preparation: A measured volume of aquarium water is taken into the test vial.

2. Addition of a reagent / titrant: You use a syringe or dropper to add reagent(s) slowly, usually drop by drop, while swirling or mixing.

3. Endpoint detection (color change): As the reagent neutralizes the carbonate / bicarbonate buffer, the solution’s color shifts (or you look for a colorless endpoint). The point at which this color shift happens corresponds to the alkalinity.

4. Calculate result: The number of drops used (or exact volume of reagent consumed) is translated using the kit instructions into a KH value (often in dKH or meq/L).

Strengths

• Relatively fine resolution / incremental steps: The ability to detect changes of ~0.3 dKH (or small meq/L shifts) is beneficial.

• Trusted in reef hobby: Many reefers use this kit and regard it as a practical, hobbyist-level solution. 

• Generous test count: Around 100 tests per kit makes it economical for routine monitoring.

Limitations

• Variability / repeatability concerns: Some users compare Salifert to other tests and report greater variability between replicates. For example, one reefer noted getting 10.2 vs 10.9 dKH on two tests minutes apart, while a Hanna device yielded 9.6 / 9.7.

• User dependency: Because the method is titration / drop-based, careful technique (especially precise drop counting and consistent mixing) is critical to reduce error.

• Subject to lighting / observation bias: The endpoint color shift must be judged by eye — lighting conditions, tube clarity, or observer perception may lead to mis-interpretation.

• Kit drift / reagent age: Over time, reagents may degrade, especially if stored poorly or exposed to heat, which can affect accuracy.

• Marine-only versus freshwater: According to one source, the marine version may give inaccurate readings if used in freshwater.

To maximize accuracy and consistency with the Salifert KH / Alkalinity Test Kit, consider the following:

• Use clean, clear vials / tubes, free of scratches or residue.

• Rinse the test vial with ambient aquarium water before using the sample to minimize contamination or dilution errors.

• Bring reagents and sample to a consistent temperature (ideally room temperature) to reduce variation.

• Accurate dosing: Use the syringe/dropper carefully, and avoid overshooting — add reagent drop by drop near the expected endpoint.

• Swirl / mix gently and consistently between additions to ensure uniform reaction.

• Count drops (or track exact volume) methodically, and use the same process each time.

• Read endpoint carefully (watch for sharp color change) and compare to instructions.

• Control lighting conditions (neutral white light, minimal glare) when interpreting color change.

• Repeat measurements to check consistency (e.g. run two replicates).

• Track trends over time rather than relying solely on single-point absolute accuracy — if your readings are consistent, you can trust change more than absolute value.

• Store reagents properly (cool, dark location, tightly sealed) to reduce degradation.

• If reagent runs out or the sample is off chart, you may need to dilute or replace.

Interpretation 

• Target ranges: In many reef systems, desirable KH (carbonate alkalinity) lies in the range of ~8–12 dKH (or ~2.8 to 4.3 meq/L) depending on protocol.

• Why it matters: Alkalinity acts as a buffer — it helps prevent wide swings in pH, and supports coral calcification (in conjunction with calcium and magnesium).

• If alkalinity is too low: You may see coral stress, slower growth, trouble maintaining calcium/alkalinity stability, and pH fluctuations.

• If alkalinity is too high: Risk of precipitation (calcium carbonate forming out of solution), especially in presence of high calcium and magnesium levels.

• Trend monitoring is key: Rather than over-focusing on exact numbers, track how your alkalinity evolves after dosing, water changes, or consumption by corals.

• Balancing with other elements: Alkalinity does not exist in isolation. You should interpret KH alongside calcium, magnesium, pH, and overall ionic balance.