• Genuine Red Sea part R21406 — the same three-reagent set supplied in the complete kit R21405.
• All three reagents in one box — Reagent A, the powdered Reagent B and the Reagent C titrant, so nothing is left half-stocked.
• 5 ppm resolution — tight enough to see a 20 ppm daily change, which is the maximum Red Sea considers safe.
• Titration to a colour change — the endpoint is a clear shift rather than a judgement call against a printed card.
• Cheaper and less wasteful than a second kit — the titrator is the expensive part and it does not wear out.

• Reagent A
• Reagent B (powder)
• Reagent C (titrant)

Together these restore the kit to its full 75-test count.

Not included: the professional titrator, the 5 mL syringe, the dosing spoon for the powdered reagent, the glass vial, the colour reference card, the instruction manual and the storage case. This is chemistry only.

If this would be your first Red Sea calcium test, buy the Red Sea Calcium Pro Testing Kit instead — without the titrator and the dosing spoon there is no way to run the test.

Red Sea publishes calcium targets as a trade-off between how fast corals grow and how well they colour up. Higher calcium and alkalinity push growth; lower figures favour coloration.

Below your target: dose Red Sea Foundation A (Calcium+), or the powdered Foundation A if you mix your own stock solution. Red Sea states a maximum safe rise of 20 ppm in one day.

Above your target: stop dosing and let coral demand pull it back. Calcium sitting high alongside high alkalinity risks precipitation, which strips both at once and coats heaters and pump impellers in white scale.

Reading correct but corals still not growing: calcium is rarely the limiting factor on its own. Test alkalinity and magnesium before you add any more calcium — low magnesium in particular makes both of the others impossible to hold.

Red Sea's guidance is to test three days after starting a new dosing regime to see how fast the tank is consuming calcium, then settle into weekly testing once the dose is dialled in.

Always check salinity first and allow ten minutes after any adjustment before drawing a sample. A tank that has just had top-up water added will read low for reasons that have nothing to do with calcium.

Level the powdered Reagent B in the dosing spoon rather than heaping it — an over-full spoon is the most common cause of a Calcium Pro result that reads high and will not reconcile with anything else.

Store all three reagents closed, between 15 and 25 °C, in the kit's case and away from prolonged light. Red Sea states the reagents are stable up to the date printed on the pack when stored that way. Keep them away from acids, oxidising compounds and bare metal.

Reagent A is the one to respect. Red Sea's safety data sheet lists sodium hydroxide at 10–25% with the signal word Danger: it may be corrosive to metals and it causes severe skin burns and eye damage. Wear eye protection and gloves, and do not decant it into an unlabelled container.

Reagent C contains EDTA at 1–5% and a trace of sodium hydroxide, carries the signal word Warning, and is an eye irritant. Reagent B is not classified as hazardous.

Keep all three out of reach of children. For eye contact, rinse with water for at least 15 minutes and seek medical attention. For skin contact, remove affected clothing and wash thoroughly with soap and water. If swallowed, do not induce vomiting — rinse the mouth and get medical advice.

Suitable for: owners of the Red Sea Calcium Pro Test Kit who have run down any of the three reagents, and reef keepers dosing a calcium supplement who need better resolution than a colour-comparison kit provides.

Not suitable for: freshwater calcium or general hardness testing. It will not serve Red Sea's older Marine Care calcium kit, which uses different chemistry, and it is not a standalone product — there is no titrator in the box.

• Red Sea Calcium Pro Testing Kit — the complete kit this refill restocks.
• Red Sea Foundation A (Calcium+) — the supplement you meter against the result.
• Red Sea Alkalinity Pro Reagent Refill — the partner parameter; calcium and alkalinity are consumed together.
• Red Sea Magnesium Pro Reagent Refill — the parameter that holds the other two up.
• Red Sea Skeletal Elements Foundation ABC+ 1 kg — all three foundation elements in one powder.
• Red Sea Reef Foundation Pro Multi Test Kit — calcium, alkalinity and magnesium in a single case.

Any manufacturer warranty Red Sea offers applies in addition to your rights under the Australian Consumer Law, which are not limited or excluded by it.

Ships as a standard small parcel. Reagent volumes are well under the threshold for dangerous goods freight.

How many tests does the Red Sea Calcium Pro reagent refill provide?

The refill restores the Calcium Pro Test Kit to its full 75-test count, which is the figure Red Sea specifies for the complete kit.

What comes in the Calcium Pro refill?

Three reagents: Reagent A, the powdered Reagent B, and Reagent C, which is the titrant. The titrator, dosing spoon, syringe, glass vial, colour card and case are not included — they come with the complete kit and are meant to last across many refills.

What is the Red Sea part number for the Calcium Pro refill?

The refill is Red Sea part R21406 and the complete kit is R21405. Red Sea's safety data sheet covers both under one document because the chemistry is identical.

My calcium reads correctly but my corals are not growing. Is the test wrong?

Usually not. Calcium, alkalinity and magnesium are consumed together, and a correct calcium figure alongside low alkalinity or low magnesium will still stall growth. Test all three before adding more calcium.

Is Reagent A dangerous?

It requires care. Red Sea classifies Reagent A with the signal word Danger — it contains 10–25% sodium hydroxide, causes severe skin burns and eye damage, and may corrode metals. Wear gloves and eye protection, keep it in its original labelled bottle, and store it out of reach of children.

Why does my result change if I heap the powder?

Reagent B is measured by volume with the supplied spoon, so a heaped spoon puts more reagent into the vial than the method assumes and shifts the endpoint. Level the spoon against the edge of the container every time and results become repeatable.