• Genuine Red Sea part R21416 — the same reagents supplied in the complete Magnesium Pro kit, not a third-party substitute.
• Restores the full 100 tests — the same test count as a new kit, which is the largest in Red Sea's Pro foundation range.
• 20 ppm resolution retained — fresh reagent is what that figure assumes; an aged bottle quietly stops delivering it.
• Keeps the hardware — the titrator and dosing spoon are built to outlast many refills, so there is no reason to buy them twice.
• Fixes disagreeing kits — when two magnesium tests give different answers, the older reagent is almost always the one that is wrong.

• Red Sea Magnesium Pro reagents, enough to restore 100 tests

This is reagent only. The titrator, dosing spoon, 5 mL syringe, glass vials, colour reference card, instruction manual and storage case all ship with the complete kit and are not repeated here.

If you do not already own the Red Sea Magnesium Pro Test Kit, buy the kit instead — on its own there is nothing in this box to titrate with, and no spoon to measure the powder step.

It fits the Red Sea Magnesium Pro Test Kit, part R21415 — the titration kit with the single-hand titrator and the dosing spoon.

It does not fit Red Sea's Marine Care magnesium test, which uses different chemistry, and it does not fit the magnesium test inside the Reef Foundation Pro Multi Test Kit unless that kit's magnesium module is the Pro test. Check the reagent bottle labels against R21416 before ordering if you are unsure which kit you own.

Level the dosing spoon every time. The powder step is measured by volume, and a heaped spoon moves the endpoint further than the 20 ppm the kit resolves to.

Rinse the vials in RO water between tests, and check salinity before you draw a sample — magnesium tracks with salinity, so a reading taken at the wrong specific gravity is misleading rather than merely imprecise.

Store the reagents closed in their original containers, cool, dry and out of direct sun, away from acids, alkalis, oxidising compounds and metals. If a fresh refill reads differently from the bottle you were finishing, trust the fresh one.

Red Sea's safety data sheet covers this refill and the complete kit together, and gives Reagent A the signal word Danger. Reagent A is classified as harmful if swallowed and harmful in contact with skin, causes severe skin burns and eye damage, and may be corrosive to metals. The named constituents are sodium tetraborate decahydrate at 1–5%, which carries a reproductive hazard classification, potassium hydroxide at 1–5% and oxalic acid at 1–5%. Reagents B and C carry no signal word.

Keep out of reach of children. In the eyes, rinse with plenty of water for at least 15 minutes and get medical attention. If swallowed, do not induce vomiting — rinse the mouth thoroughly with water if the person is conscious, and get medical attention.

Suitable for: anyone who already owns a Red Sea Magnesium Pro Test Kit and has run the reagents down or out of date.

Not suitable for: a first purchase — there is no titrator, spoon or glassware in this box. It is also not for freshwater use, and it will not measure calcium or alkalinity.

• Red Sea Magnesium Pro Testing Kit — the kit this refill restocks, if you need the titrator too.
• Red Sea Foundation C (Magnesium) — the supplement you dose against the reading.
• Red Sea Alkalinity Pro Reagent Refill — the reagent that empties fastest of the three.
• Red Sea Calcium Pro Reagent Refill — the third foundation parameter.
• Red Sea Reef Foundation ABC 3 Pack — all three supplements together.
• Red Sea range — the rest of the Reef Care testing and supplement program.

Reagents are consumables and carry no service warranty. 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. The reagent volumes are well under the threshold for dangerous goods handling, so no special freight applies.

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

The refill restores the Magnesium Pro Test Kit to its full 100-test count, which is the number of tests Red Sea specifies for the complete kit.

What is the Red Sea part number for this refill?

The refill is Red Sea part R21416. The complete kit is R21415. Both are covered by the same Red Sea safety data sheet, because the refill supplies the same reagents.

Does the refill include the titrator or the dosing spoon?

No. The refill is reagent only. The titrator, dosing spoon, syringe, glass vials, colour card, manual and case all come with the complete Magnesium Pro Test Kit and are designed to be reused across many refills.

Do the reagents expire?

Yes, and magnesium reagent is more likely than most to age out before it empties, because magnesium is typically tested fortnightly rather than several times a week. Check the date on the pack, and replace the reagent if a result stops agreeing with your other tests.

Will this refill work in Red Sea's Marine Care magnesium test?

No. The Marine Care magnesium test uses different chemistry from the Pro titration kit, and the reagents are not interchangeable. This refill is specific to the Magnesium Pro Test Kit, part R21415.