What is two-part dosing in a reef tank?

Two-part dosing is a method of replacing the calcium and alkalinity that growing corals consume, using two separate solutions dosed in equal amounts. Part A is a calcium source, usually calcium chloride. Part B is an alkalinity source, usually sodium carbonate (soda ash) or sodium bicarbonate. They are kept separate because mixing them in concentrated form causes calcium carbonate to precipitate out immediately. Dosed separately into a well-mixed tank, they replace both parameters in the same balanced ratio that corals remove them.

When should I start two-part dosing?

Start when water changes can no longer keep alkalinity stable. A practical test is to measure alkalinity, wait 24 hours without dosing anything, and measure again. If you are losing less than about 0.5 dKH per day, regular water changes will usually cover it. Above roughly 0.5 dKH per day, you need daily supplementation, and two-part is the most controllable way to provide it. New tanks with small frags rarely need it; the same tank a year later almost always does.

How do I calculate how much two-part to dose?

Measure your daily consumption first. Test alkalinity, wait 24 hours with no dosing, and test again — the difference is your daily demand. Then dose the amount of Part B that raises your tank volume by exactly that much, and an equal volume of Part A. Follow your product's stated strength, or use a dosing calculator to convert dKH into millilitres or grams for your specific tank volume. Retest after three days and adjust in small steps.

Can I dose both parts at the same time?

No. Dosing Part A and Part B together, or into the same spot within a short window, causes calcium carbonate to precipitate before it can disperse. You lose the supplement, you can cloud the water, and precipitate can coat pumps and heaters. Separate the two doses by at least an hour, and add them at different high-flow points in the sump or display.

Does two-part dosing raise salinity?

Slowly, yes. Both parts leave behind sodium and chloride ions that corals do not consume, so over months of heavy dosing they accumulate as ordinary salt. For most tanks routine water changes remove this faster than it builds up. Only very high-consumption systems dosing large daily volumes need to worry, and those tanks are usually better served by a calcium reactor or kalkwasser.

Is two-part dosing better than kalkwasser or a calcium reactor?

It depends on demand. Two-part is the easiest to start, the easiest to adjust, and the best fit for low to moderate consumption. Kalkwasser is cheap and helps hold pH up, but its capacity is capped by how much evaporation you get. A calcium reactor handles the highest demand at the lowest running cost, but costs more upfront and requires tuning and CO2. Most reef keepers start on two-part and only move on when the daily volume becomes impractical.

Two-Part Dosing for Reef Tanks: Where to Start

Your alkalinity reads 8.5 dKH on Sunday. By Wednesday it’s 7.4. You do a water change, it comes back up, and by the following Wednesday it’s low again.

That is not a problem with your test kit or your salt mix. It is your corals doing exactly what they are supposed to do — and it is the point at which most reef keepers start two-part dosing.

What Two-Part Dosing Actually Is

Corals build their skeletons out of calcium carbonate. To do that they pull two things out of your water continuously: calcium ions and carbonate, which is what your alkalinity test is measuring. They remove them in a fixed ratio, roughly 20 ppm of calcium for every 1 dKH of alkalinity.

Two-part dosing replaces both in that same ratio, using two separate bottles:

Part A — a calcium source, almost always calcium chloride
Part B — an alkalinity source, either sodium carbonate (soda ash) or sodium bicarbonate (baking soda)

They live in separate containers for a very practical reason. Mix concentrated calcium and concentrated carbonate together and they immediately form solid calcium carbonate — the same chalky precipitate that fouls heaters and pump impellers. Diluted into a few hundred litres of moving water, they stay in solution and your corals get to use them.

That is the whole concept. Everything else is bookkeeping.

Do You Actually Need It Yet?

Plenty of tanks get dosed before they need it, which mostly succeeds in making the water chemistry harder to read.

Run this test instead. Measure alkalinity, then leave the tank alone for 24 hours — no dosing, no water change. Measure again at the same time the next day. The difference is your daily consumption.

Daily alkalinity loss	What it means
Under 0.3 dKH	Very low demand. Water changes are fine.
0.3 – 0.5 dKH	Borderline. Watch it; you’ll likely need dosing soon.
0.5 – 1.5 dKH	Normal for a growing mixed reef. Start dosing.
Over 1.5 dKH	High demand. Dose daily, and consider automation.

A tank with a handful of frags might sit under 0.3 dKH for a year. That same tank with grown-out colonies will comfortably clear 1 dKH per day. Demand rises as coral mass rises, so this is a number worth re-checking every few months rather than measuring once and forgetting.

If you are still working out what your baseline numbers should be, our guide to calcium, alkalinity and magnesium covers target ranges and how the three parameters interact.

Fix Magnesium Before You Start

This is the step people skip, and it costs them weeks.

Magnesium is what keeps calcium and carbonate in solution at the concentrations we run. When magnesium is low — below about 1,250 ppm — calcium and alkalinity will not hold no matter how faithfully you dose. You will add Part A and Part B, watch the numbers rise, and find them back down the next morning because the supplement precipitated out instead of staying available.

Get magnesium to roughly 1,300–1,400 ppm first. Raise it no faster than 50 ppm per day. Then start on calcium and alkalinity.

Getting to Target vs. Staying at Target

These are two different jobs and mixing them up is the most common early mistake.

Correcting a deficit is a one-off. If your alkalinity is at 6.5 and you want 8.5, that is a 2 dKH correction, and it must be spread over at least two days because raising alkalinity faster than 1 dKH per day causes tissue necrosis in SPS corals. This is a temporary, larger dose.

Maintenance dosing is the ongoing job — replacing exactly what gets consumed each day so the number never moves at all.

Do the correction first, get stable at your target, and only then work out your maintenance dose. Trying to do both at once means you cannot tell which part of the change came from which, and you end up chasing the number up and down for weeks.

Our alkalinity calculator handles the correction side — enter your current level, your target, your tank volume and your supplement, and it returns the dose and splits it across days if the correction is too large to do safely at once.

Working Out Your Daily Dose

Once you are stable at target:

Know your consumption. From the 24-hour test above — say 0.8 dKH per day.
Convert that to volume. Every two-part product states how much it raises alkalinity per unit volume of tank water. Use that figure to work out the millilitres that equal 0.8 dKH in your system.
Dose Part A and Part B in equal amounts. They are formulated to be balanced, so equal volumes maintain the calcium-to-alkalinity ratio your corals are consuming.
Split the doses across the day if you can. Four small doses hold the parameter far steadier than one large one, which is the main argument for a dosing pump.
Retest after three days. Adjust in small increments. If alkalinity is creeping up, dose slightly less; if it is drifting down, dose slightly more.

Then leave it alone. The instinct to adjust after every single test is what produces the sawtooth alkalinity graphs that stress corals more than being slightly off-target ever would.

Manual dosing vs. a dosing pump

Manual dosing works perfectly well and costs nothing beyond the supplements. It also means you personally have to show up twice a day, every day, including the days you are away.

A dosing pump removes that dependency and lets you split the daily amount into many small additions, which keeps alkalinity flatter. For any tank consuming more than about 1 dKH per day, it stops being a luxury fairly quickly.

Whichever you use, keep dosing points apart — Part A at one end of the sump, Part B at the other, separated by at least an hour.

What Goes Wrong

Dosing both parts together. Cloudy water, wasted supplement, and precipitate on your equipment. Separate them in time and location.

Chasing test results. Test kits have real margins of error. A reading of 8.3 followed by 8.6 is very likely the same alkalinity measured twice, not a trend. Change your dose based on a direction sustained over several days, not a single test.

Ignoring magnesium. If your numbers refuse to hold, check magnesium before you increase anything else.

Assuming demand is static. Consumption climbs as corals grow. The dose that was correct in spring will be short by autumn. Re-run the 24-hour consumption test quarterly.

Letting salinity drift. Both parts add sodium chloride that nothing consumes. Routine water changes clear it in most tanks — but if you are dosing heavily and salinity is creeping up, that is the cause.

If your alkalinity is dropping faster than dosing can explain, the problem is usually somewhere other than the dosing schedule. Our guide to troubleshooting low alkalinity works through the other causes.

When to Move On From Two-Part

Two-part scales well until it doesn’t. Once you are dosing very large daily volumes, you are buying and hauling a lot of liquid, adding a lot of inert salt, and refilling containers constantly.

At that stage the alternatives start making sense. Kalkwasser is inexpensive and helps hold pH up, but its ceiling is set by your evaporation rate. A calcium reactor handles the highest demand at the lowest ongoing cost, at the price of a bigger upfront spend, a CO2 cylinder, and a tuning process.

There is no rush. Plenty of successful SPS tanks run on two-part indefinitely. The transition is worth making when the logistics become annoying, not because a reactor is inherently better.

The Bottom Line

Two-part dosing is a straightforward answer to a straightforward problem: corals consume calcium and alkalinity every day, and something has to put them back.

Measure your consumption before you start. Fix magnesium first. Separate the correction from the maintenance. Then dose consistently and resist the urge to fiddle.

Stability is the goal — not a particular number on a test kit. A tank held rock-steady at 7.8 dKH will always outperform one bouncing between 7 and 10 in pursuit of the perfect reading.