• Genuine Red Sea part R21411 — the same titrant supplied in the complete KH/Alkalinity Pro kit, not a third-party substitute.
• Restores the full 75 tests — a whole kit's worth of testing without buying a second titrator.
• 0.05 meq/L (0.14 dKH) resolution — fine enough to catch a drift while it is still a drift, not after the corals have browned out.
• Titration, not colour matching — you read a number off a syringe scale instead of holding a vial against a printed card under kitchen lighting.
• Fresh, in-date chemistry — the single most common cause of two test kits disagreeing is one of them being old.

• Red Sea KH/Alkalinity Pro titrant reagent, enough for 75 tests

Not included: the titrator, the 10 mL syringe, the glass vial, the colour reference card, the instruction manual, the storage case, and the 100 mL sample of Foundation B that ships with the complete kit. This is reagent only.

If you do not already own the Red Sea KH/Alkalinity Pro Test Kit, buy the kit rather than this refill — on its own there is nothing here to titrate with.

There is no single correct number. Red Sea's Reef Care programme publishes alkalinity targets as a trade-off between growth rate and coloration, and the figure you pick depends on which you are chasing.

Stability matters more than the number itself. Pick a target, hold it, and change it slowly if you change your mind.

Below your target: dose Red Sea Foundation B (KH/Alkalinity), which is the supplement this test kit exists to meter. Red Sea states a maximum safe increase of 1.4 dKH (0.5 meq/L) in a single day — going faster stresses corals rather than helping them.

Above your target: stop dosing and let consumption bring it down, or dilute with a water change. Do not chase it downwards with acid.

Drifting when nothing has changed: calcium, alkalinity and magnesium move as a set. A stubborn alkalinity reading usually means magnesium is low, so test all three before adjusting any of them.

Check and correct salinity before testing anything. Red Sea's guidance is to allow ten minutes after a salinity adjustment for the tank to stabilise before you draw a sample.

Store the reagent closed, between 15 and 25 °C, and keep it in the kit's plastic case — prolonged light exposure is what degrades it. Red Sea states the reagent is stable up to the date printed on the pack when stored that way.

If a fresh refill reads noticeably differently from your old bottle, trust the fresh one. That gap is the reagent you were still using telling you it had aged out.

The titrant carries a GHS signal word of Warning. Red Sea's safety data sheet lists monoethylene glycol (an antifreeze-type solvent) at 50–75%, classified as harmful if swallowed, and hydrochloric acid at under 1%, which can cause severe skin and eye burns and may irritate the airways.

Keep it out of reach of children. Do not eat, drink or smoke while using it. If it gets in your eyes, rinse with water for at least 15 minutes and seek medical attention. Do not induce vomiting if swallowed — rinse the mouth and get medical advice.

Suitable for: anyone who already owns a Red Sea KH/Alkalinity Pro Test Kit and has run down the reagent, and anyone keeping a marine or reef aquarium who needs alkalinity resolution finer than a colour-card kit can give.

Not suitable for: freshwater KH testing, where a low-cost colour kit is entirely adequate. It is also not a substitute for the complete kit — there is no titrator or glassware in this box — and it will not fit Red Sea's older Marine Care alkalinity kit, which uses different chemistry.

• Red Sea KH/Alkalinity Pro Testing Kit — the kit this refill restocks, if you need the titrator too.
• Red Sea Foundation B (KH/Alkalinity) — the buffer you dose against the reading.
• Red Sea Calcium Pro Reagent Refill — the second of the three foundation parameters.
• Red Sea Magnesium Pro Reagent Refill — the third, and the one that quietly explains most alkalinity problems.
• Red Sea Reef Foundation ABC 3 Pack — all three supplements together.
• Red Sea Reef Foundation Pro Multi Test Kit — calcium, alkalinity and magnesium in one 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. The reagent volume is well under the threshold for dangerous goods handling, so no special freight applies.

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

The refill restores the KH/Alkalinity Pro Test Kit to its full 75-test count, which is the number of tests Red Sea specifies for the complete kit.

Does the refill include the titrator?

No. The refill is reagent only. The titrator, syringe, glass vial, colour card, manual and case all come with the complete KH/Alkalinity Pro Test Kit and are designed to be kept and reused across many refills.

What is the Red Sea part number for the KH/Alkalinity Pro refill?

The refill is Red Sea part R21411. The complete kit is R21410. Both are covered by the same Red Sea safety data sheet, because the refill supplies the same titrant.

How accurate is the Red Sea Alkalinity Pro test?

Red Sea specifies a resolution of 0.05 meq/L, which is 0.14 dKH. Because it is a titration rather than a colour comparison, the range is effectively unlimited — you titrate to an endpoint instead of matching against a fixed printed scale.

Do the reagents expire?

Yes. Red Sea states the reagent is stable up to the date printed on the pack when stored closed between 15 and 25 °C and kept in its case away from prolonged light. Reagent age is the usual explanation when two alkalinity kits disagree with each other.

Can I use this refill in a freshwater aquarium?

The chemistry will measure carbonate hardness in fresh water, but this kit is built and calibrated around the resolution a reef needs. For a freshwater tank a standard colour-comparison KH kit is cheaper and perfectly adequate.