Reef Tank Dosing: Two-Part, Kalkwasser, and Calcium Reactors
Once your reef has enough stony coral, water changes alone cannot keep up with calcium and alkalinity consumption. Here is how each dosing method works, when to use it, and what happens when things go wrong.
Stony corals — both LPS and SPS — build their skeletons from calcium carbonate. To do that, they constantly pull calcium and alkalinity out of the water column. In a tank with a handful of frags, weekly water changes with quality salt mix can replace what they consume. But as your coral grows and your collection expands, the math stops working in your favor. A tank with 20 or 30 SPS colonies can consume more alkalinity in a day than a 10% water change replaces in a week.
Dosing is how you bridge that gap. There are three main approaches, each with different cost profiles, complexity levels, and scalability. Understanding them helps you choose the right method for your system now and plan for where it will be in two years.
When to Start Dosing
Test your alkalinity on two consecutive days without a water change in between. If it drops more than 0.5 dKH in 24 hours, your coral is consuming faster than passive maintenance can replace. Do the same for calcium — a drop of more than 10 ppm per day is the threshold. Either of these signals means it is time to start supplementing.
Target parameters before you start: alkalinity 8–10 dKH, calcium 400–450 ppm, magnesium 1250–1350 ppm. Get to these targets with water changes first, then begin dosing to maintain them. Never try to use dosing to fix a parameter crash — correct the crash with water changes, stabilise, then begin the dosing routine.
Method 1 — Two-Part Dosing
Two-part is the most common starting point. It involves two separate liquid solutions: Part A raises alkalinity using sodium bicarbonate or sodium carbonate, and Part B raises calcium using calcium chloride. They are dosed in equal volumes, typically via a dosing pump running several times throughout the day. Splitting the daily dose across multiple small additions prevents parameter spikes between doses.
Pros
- Simple to set up — no CO2, no reactor, no complex plumbing.
- Highly precise. You control exactly how much is added and when.
- Easy to dial in: test, adjust dose volume, test again.
- Ideal for low-to-medium coral demand — most mixed reef tanks fall in this range.
Cons
- Ongoing cost scales with consumption. As your coral grows and demand increases, you go through solution faster. At high SPS demand, the per-litre cost becomes significant.
- Calcium chloride (Part B) adds chloride ions to the water with every dose. At very high dose volumes this can push ionic balance out of natural seawater ratios over time. Periodic larger water changes help reset ionic balance.
Recommended Products
- BRS bulk two-part: The best value option by a wide margin. Buying the dry powder in bulk and mixing your own solutions reduces cost dramatically compared to pre-mixed retail bottles. BRS publishes exact mixing ratios. Pairs naturally with BRS dosing pumps.
- Two Little Fishies C-Balance: A pre-mixed option with a balanced formulation. More convenient than mixing from powder; more expensive per litre. Good for smaller systems where you are not going through large volumes.
Method 2 — Kalkwasser
Kalkwasser (German for "lime water") is a saturated solution of calcium hydroxide — Ca(OH)₂ — mixed into fresh RO/DI water and dripped into the tank, typically via the ATO reservoir. When the tank evaporates water, the ATO tops off with kalkwasser-dosed freshwater instead of plain RO/DI, delivering calcium and alkalinity in every top-off dose.
Kalkwasser has a useful secondary effect: calcium hydroxide is strongly alkaline, and dripping it into the tank raises pH. This makes it excellent for addressing the nighttime pH dip that planted or algae-scrubber systems reduce but most sumps cannot fully eliminate.
Pros
- Very cheap — calcium hydroxide powder costs almost nothing per gram.
- Raises both calcium and alkalinity simultaneously in roughly balanced proportions.
- Passively raises pH, counteracting the nighttime dip from respiration.
- No dosing pump required if you integrate it into your existing ATO line.
Cons
- Dose is limited by evaporation rate. You can only drip as much kalkwasser as your tank evaporates per day. In a well-covered, low-evaporation system, this may not be enough to meet coral demand.
- Overdosing raises pH above safe levels (above 8.4 consistently can stress coral and fish). The ATO rate places a natural ceiling on dosing, but kalk-dedicated ATO lines need careful calibration.
- Kalkwasser solution is caustic. Handle carefully, do not let it contact skin or eyes.
Kalkwasser is most effective as a supplement to two-part dosing rather than a standalone method for high-demand systems. It handles base demand and pH, while two-part covers the deficit at peak growth periods.
Method 3 — Calcium Reactor
A calcium reactor is the most automated dosing method for large or SPS-dominant systems. CO2 is injected into a sealed chamber packed with aragonite media — the same calcium carbonate mineral that coral skeletons are made from. The CO2 acidifies the water inside the chamber, which dissolves the aragonite and releases calcium, alkalinity, and magnesium into solution. This mineral-rich effluent then drips back into the sump at a controlled rate, replenishing exactly what the coral consumed.
The elegance of the calcium reactor is that it replenishes elements in approximately natural seawater ratios, since it is dissolving actual reef mineral. Magnesium, strontium, and trace elements come along for the ride in addition to calcium and alkalinity.
Pros
- Handles very high coral demand without scaling cost — you add media, not liquid solution.
- Replenishes calcium, alkalinity, and magnesium in natural ratios simultaneously.
- Once dialled in, it is genuinely low-maintenance. Refill media every few months.
- No ionic imbalance issues from repeated chloride additions.
Cons
- High upfront cost. A quality reactor, CO2 cylinder, regulator, and associated plumbing runs $300–600 to set up properly. Budget options exist but CO2 regulation quality matters — a failed regulator can dump CO2 and crash your tank's pH.
- CO2 injection lowers the pH of the effluent. If the reactor's effluent pH is too low, it drags down the display tank pH. A second chamber (dual-stage reactor) off-gasses CO2 from the effluent before it enters the sump and largely solves this.
- Takes time and testing to dial in correctly. Effluent alkalinity, flow rate, and CO2 injection rate all interact. Expect a few weeks of adjustment before it runs stably.
How to Choose
Two-part is the right starting point for almost every reefer. It is easy to set up, easy to understand, and works well through most of a tank's life. Kalkwasser through the ATO is a cost-effective supplement that improves pH and reduces how much two-part you need. A calcium reactor becomes worth the investment when your daily calcium and alkalinity consumption is so high that two-part becomes expensive and your dosing pump is running constantly.
When Things Go Wrong
Alkalinity swings kill coral faster than almost any other parameter problem. A rapid swing of 2 or more dKH in either direction can cause RTN (rapid tissue necrosis) across an entire SPS display. The rules:
- Never change alkalinity by more than 0.5–1 dKH per day when correcting a problem.
- Always test before adjusting. Chasing a number without knowing the current value risks overcorrection.
- Never chase calcium without checking magnesium. Magnesium blocks calcium uptake when depleted — if calcium refuses to hold even after dosing, magnesium is likely the culprit. Target 1250–1350 ppm.
- If your alk consumption suddenly spikes, look for a parameter problem in the tank before increasing your dose. Rapid consumption can indicate pH-driven precipitation (white snow in the sump) or a dying coral leaching from the opposite direction.
