The Salifert Calcium Test Kit is used to measure the concentration of calcium ions (Ca²⁺) in aquarium water, especially in marine / reef systems.

Calcium is a critical element for corals, mollusks, and other invertebrates that build calcium carbonate skeletons or shells. Keeping calcium in the correct range helps support healthy growth, skeletal formation, and overall stability of the reef system.

In reef-keeping, calcium is part of the “major elements” (along with alkalinity and magnesium) whose balance is essential for stable water chemistry.

Calcium is a major constituent of calcareous algae, skeletal material of hard corals and the skeletal needles of soft/leather corals. Calcium also fulfills many biological functions.

Quite often a too low calcium concentration retards coral and calcareous algae growth.

NSW contains 400 – 450 mg/L calcium. Which is also the value we should strive for. For a stable aquarium environment the fluctuation in calcium content should preferably not more than approx. 15 mg/L. This automatically means that the calcium test should be able to monitor comparable fluctuations.

Conclusion:

In a healthy aquarium growth and multiplication of corals, desired calcareous algae and other organisms results in calcium depletion. Because of the importance of calcium timely corrective measures would have to be taken. For that purpose a proper calcium additive together with an accurate test kit are required.

Our calcium test is capable in resolving the calcium content in 10 mg/L steps. The color change is sharp and facilitates the detection of small but highly significant fluctuations in the calcium concentration. Salifert is the only one in offering a true accuracy and precision as claimed.

Salifert's Coral Calcium additive is manufactured using a very special procedure. This results in a highly concentrated additive that not only makes calcium addition easy, it also avoids that the ionic balance is disturbed when used in combination with our KH + pH Buffer or any other sodium based buffer. Common additives, which contain only calcium chloride or improperly use of kalkwasser (calcium hydroxide), can upset the ionic balance resulting in aquarium water very different from NSW.

A typical Salifert Calcium Test Kit includes:

• Reagent solution(s)

• Measuring syringe or dropper(s) for precise reagent addition

• A test vial / tube

• Instruction manual / color chart or titration table

• Packaging / holder for the components

• Enough reagent for ~50 tests (depending on calcium concentration and reagent usage)

Because reagent usage per test depends on how much reagent is consumed (which depends on calcium level), the actual number of tests may vary.

Principle & Chemistry

• The Salifert Calcium kit works via a colorimetric / titration-based method (in some versions called “Profi Test”) in which a reagent is added to a known volume of sample water until a color endpoint is reached.

• As you add the reagent, it reacts with the Ca²⁺ ions, and at a certain point (the endpoint) the solution shifts color. That shift indicates the amount of reagent used, which is proportional to the calcium concentration.

• The kit’s calibration / chart allows you to convert reagent consumption (drops or volume) into a calcium concentration (in mg/L or ppm).

• The color change is described as “sharp,” enabling the detection of relatively small changes in calcium.

• The kit’s resolution is in small increments. Some sources say that the kit measures in steps of 5 mg/L, with an accuracy of ±10 mg/L.

Strengths

• Fine resolution: The ability to detect ~5 mg/L increments gives the user sensitivity to small changes in calcium.

• Sharp color changes: The kit is designed for distinct color shifts so the endpoint is easier to see.

• Widely used in reef community: Many hobbyists use Salifert calcium tests, so there is community experience and troubleshooting advice.

• Reasonable test count for many users: ~50 tests is sufficient for regular monitoring in typical reef tanks.

Limitations 

• User error / technique sensitivity: As with any titration / drop test, the accuracy depends heavily on consistent technique, precise drops, mixing, and endpoint judgment.

• Subjectivity of color endpoint: The endpoint color shift must be judged by eye. Ambient lighting, tube clarity, or observer bias can affect the result.

• Reagent usage variance: In tanks with higher calcium levels or more reagent required near the endpoint, you may consume more reagent per test, potentially reducing the total number of tests from the kit.

• Reagent stability: Over time, reagents may degrade or become contaminated, affecting accuracy.

• Limit to the kit’s calibration range: If calcium is extremely high or low, you may fall outside the calibrated range and need to dilute or use alternate methods.

To get the most accurate and repeatable results with the Salifert Calcium Test Kit:

• Use clean, scratch-free glass vials — scratches or residue can distort color viewing.

• Rinse vials with sample water before measurement to minimize contamination.

• Bring sample and reagents to ambient temperature (or consistent temperature) to minimize thermal effects.

• Add reagent drop by drop, especially as you near the expected endpoint.

• Swirl / mix gently and consistently between drops to ensure homogeneous reaction.

• Work methodically and count drops carefully (or measure reagent volume precisely).

• Observe the color change carefully — the “sharp” change is the point to stop.

• Perform duplicate tests (run two replicates) to check consistency.

• Conduct tests under consistent lighting conditions (neutral white light, minimal glare).

• Record related parameters (e.g. alkalinity, magnesium, temperature, salinity) alongside calcium reading to interpret results in context.

• Monitor trends over time, rather than relying solely on single absolute values.

• Store reagents properly (cool, sealed, dark environment) to prolong stability.

• If your sample yields an endpoint beyond the test’s range, consider dilution and adjusting the result accordingly.

Interpretation of Calcium Test Kit

• Target calcium ranges: In many reef systems, aquarists aim for calcium in the range of ~380 to 450 ppm (mg/L), though target values may vary depending on specific reef protocols and system design.

• Balance with alkalinity & magnesium: Calcium does not act alone: stable reef systems require calcium, alkalinity, and magnesium to be balanced. If calcium is too high relative to alkalinity or magnesium, precipitation may occur.

• If calcium is too low: You may observe slowed coral growth, impaired skeletal building, weak structure, or inability to maintain stable parameters.

• If too high: Risk of precipitation (especially of calcium carbonate) which can reduce available alkalinity and destabilize water chemistry.

• Trend monitoring more useful than absolute value: Track how calcium changes over days/weeks, and how it responds to dosing, water changes, and consumption by corals.

• Adjust dosing accordingly: Use your test results to guide calcium supplement dosing (e.g. two-part calcium/alkalinity dosing systems, kalkwasser, kalk reactors, etc.).

• Check in the context of other water parameters: A reading in isolation is less meaningful than calcium in relation to alkalinity, magnesium, pH, and overall ionic balance.