Calcium and alkalinity are the two parameters I watch most closely in my reef tank - everything from SPS coral growth to the simple stability of your water chemistry depends on keeping both in range. Once your corals start consuming more than water changes can replenish, you need a dosing strategy. The two most common options for hobbyists are two-part dosing and kalkwasser (calcium hydroxide), and I have used both at different stages of my reef keeping journey.

Safety Note: Kalkwasser is calcium hydroxide with a pH above 12 - it will cause chemical burns on skin and serious damage to eyes. Always wear nitrile gloves and safety glasses when mixing or handling kalk powder. Never add kalkwasser directly to your display tank or dump it in all at once; it must be dripped slowly or dosed at a controlled rate. Two-part solutions are less caustic but still irritating - rinse skin contact immediately and keep both chemicals away from children and pets.

Why You Need to Dose Calcium and Alkalinity

Corals - especially stony corals like LPS and SPS - use calcium and alkalinity to build their skeletons. Every time your corals grow, they pull both from the water column. Target ranges for most reef tanks are calcium at 400-450 ppm and alkalinity at 8-9 dKH. In a lightly stocked FOWLR tank or a system with only a few soft corals, regular water changes (10-15% per week) may be enough to maintain these levels. Once you have a serious coral population, water changes alone often cannot keep up.

The two methods covered in this guide - two-part and kalkwasser - are DIY-friendly, reliable, and do not require a calcium reactor. A calcium reactor is the third option and the best choice for heavily stocked SPS systems, but it is a bigger investment and covers a separate guide. If you are running a mixed reef or an LPS-dominant tank, two-part and kalkwasser are both worth considering.

What Is Two-Part Dosing?

Two-part dosing uses two separate liquid solutions - one to raise calcium and one to raise alkalinity - dosed in equal volumes. The most common commercial products are BRS Two-Part (calcium chloride and sodium bicarbonate) and similar offerings from other reef suppliers. You can also mix your own solutions at lower cost using bulk chemicals.

The reason it is called “two-part” is that you cannot mix the calcium and alkalinity solutions together before dosing - they would react and precipitate out of solution. Each part is dosed separately, usually through a dosing pump set to run a small amount multiple times per day. Consistent dosing frequency matters more than total daily volume; spreading doses across 4-6 increments per day keeps parameters more stable than one large daily dose.

I use a dosing pump set to dose 10 ml of each part four times a day in my 90-gallon mixed reef. That works out to roughly 40 ml per day of each solution - which cost me about $0.30 per day using bulk BRS chemicals. BRS two-part dosing chemicals are widely available and the most common starting point for hobbyists.

Advantages of Two-Part Dosing

Two-part dosing is the most accessible option for new reef keepers. Setup is straightforward: buy a dosing pump, set the schedule, and dial in the volume over a few weeks of testing. There is no mixing risk, no pH concerns, and it works equally well on any tank size. When demand changes - as it does when corals grow or you add new frags - you just increase the dose rate.

The downside is ongoing cost. Two-part solutions are more expensive per unit of calcium and alkalinity delivered than either kalkwasser or a calcium reactor. For a lightly to moderately stocked tank this is not a significant concern, but in a heavily stocked SPS system you can burn through two-part solution quickly.

Two-Part Dosing Requirements

  • Calcium target: 400-450 ppm
  • Alkalinity target: 8-9 dKH (some SPS keepers push to 9-10 dKH)
  • Magnesium: Keep at 1250-1350 ppm - low magnesium prevents calcium and alkalinity from stabilizing. You may need a separate magnesium supplement
  • Testing frequency: Test calcium and alkalinity 2-3 times per week when dialing in, then weekly once stable

For managing your overall reef tank water chemistry, the key is testing consistently rather than reacting to a single data point.

What Is Kalkwasser?

Kalkwasser is the German word for “lime water” - it is a saturated solution of calcium hydroxide in fresh top-off water. When you add kalkwasser to your tank, you deliver both calcium and hydroxide ions. The hydroxide raises pH and reacts with CO2 in the water, which indirectly raises alkalinity.

The most common way to use kalkwasser in a home reef is to mix kalk powder into your ATO (automatic top-off) reservoir. As the tank evaporates water throughout the day, the ATO system doses kalkwasser instead of plain RO/DI water. This passive delivery method works well for tanks with moderate calcium and alkalinity demand - the kalk replenishes both parameters as evaporation occurs.

In my experience, kalkwasser works best as a base additive for small to medium mixed reefs, not as the sole solution for high-demand SPS tanks. A standard kalk solution adds roughly 20 ppm of calcium and around 1 meq/L of alkalinity per liter dosed - if your tank evaporates 2 liters per day, that is the ceiling for what kalk can deliver without overdosing.

Kalkwasser powder is inexpensive compared to two-part solution. A 1 kg bag of calcium hydroxide costs $15-30 and can last several months in a 75-gallon mixed reef.

Advantages of Kalkwasser

Cost is the primary advantage of kalkwasser. Once you account for mixing and ATO delivery, kalk is significantly cheaper per unit of calcium and alkalinity than two-part solutions. Kalk also raises pH as a side effect - useful if your tank runs pH on the lower end due to CO2-heavy indoor air.

A secondary benefit is that kalkwasser precipitates phosphate. The high pH of a saturated kalk solution causes phosphate to bind and fall out of solution, which helps control one of the main drivers of algae growth. I have found this makes a noticeable difference in nuisance algae levels when switching from two-part to kalk, assuming your other nutrient sources are controlled.

Limitations of Kalkwasser

Kalk delivery is limited by evaporation rate. If your tank evaporates 1 liter per day, that is the maximum volume of kalk solution you can dose without adding extra water. In a heavily stocked SPS system consuming large amounts of calcium and alkalinity, kalk through the ATO often cannot keep up - you will need to supplement with two-part or eventually move to a calcium reactor.

pH swings are another concern. Adding too much kalkwasser too quickly causes a rapid pH spike that stresses corals and other livestock. Kalk should never be poured directly into the tank - it must be dripped or delivered slowly through a properly configured ATO. I set my ATO to dose in short bursts (no more than 250 ml at a time) rather than filling a large volume at once.

Cost Comparison: Two-Part vs Kalkwasser

For a 75-gallon mixed reef with moderate coral coverage:

Two-part dosing (bulk BRS chemicals):

  • Calcium chloride (dihydrate): ~$30 for 10 lbs
  • Baked baking soda (sodium bicarbonate): ~$20 for 10 lbs
  • Estimated cost: $0.15-0.40 per day depending on demand
  • Setup cost: $60-120 for a quality dosing pump

Kalkwasser:

  • Calcium hydroxide: $15-30 per kg
  • Estimated cost: $0.05-0.15 per day
  • Setup cost: $30-80 for a Kalk stirrer or modified ATO reservoir

Kalk wins on ongoing cost by a wide margin. The trade-off is that two-part is more flexible and scales to higher demand without risk of overdosing.

Which Method Is Right for Your Tank?

Choose two-part dosing if:

  • You are new to reef keeping and want a simple, controllable system
  • Your tank has high or growing coral demand and you need the flexibility to increase dose rates
  • You have a heavily stocked SPS or mixed reef where evaporation alone limits what kalk can deliver
  • You want precise, independent control over calcium and alkalinity

Choose kalkwasser if:

  • You have a lightly to moderately stocked mixed reef or LPS system
  • You already have an ATO system and want to upgrade it cheaply
  • You want to reduce phosphate passively
  • You want to support pH on a tank running low pH due to indoor air quality
  • Cost is a priority

For many reef keepers, the best long-term approach is using both together. I currently run kalkwasser through my ATO for the baseline daily top-off, and I have a two-part setup on standby to supplement when alkalinity drops faster than evaporation can cover. This combination reduces two-part consumption while keeping full control. An automatic dosing pump for the two-part side runs about $60-100 and pays for itself quickly in reduced two-part usage.

Setting Up Two-Part Dosing: Quick Start

  1. Measure your baseline - Test calcium and alkalinity daily for 3 days without dosing to establish your tank’s natural consumption rate
  2. Calculate starting dose - A common starting point for a 75-gallon system is 5-10 ml per day of each part, then adjust
  3. Spread the dose - Set your pump to 4-6 increments per day rather than one large daily dose
  4. Retest after 3-4 days - Adjust up or down by 10-20% at a time until parameters hold steady
  5. Check magnesium monthly - If calcium and alkalinity will not stay up despite dosing, low magnesium is usually the cause

A detailed look at how water chemistry fits into your overall reef tank maintenance schedule can help you build testing into a consistent routine.

Setting Up Kalkwasser Through an ATO: Quick Start

  1. Mix kalk solution - Add 1-2 teaspoons of calcium hydroxide powder per gallon of RO/DI water. Saturated solution forms at about 2 grams per liter; undissolved powder settles and is harmless
  2. Never shake or agitate - Pouring or pumping from above the settled powder avoids introducing excess calcium hydroxide into your tank
  3. Set ATO dose rate conservatively - Start with small doses (under 200 ml at a time) and monitor pH after doses. Your tank pH should not spike above 8.4-8.5 from a single ATO top-off
  4. Test calcium and alkalinity weekly - If demand exceeds what kalk through the ATO can supply, supplement with small amounts of two-part until you find the right balance

The chemistry of kalk is well-documented in Randy Holmes-Farley’s reef chemistry series at Advanced Aquarist, which is one of the most thorough non-commercial resources on the subject.

FAQ

Can I use two-part and kalkwasser at the same time?

Yes - this is a common and effective approach. Use kalkwasser through your ATO for the baseline top-off, and use two-part dosing to supplement when demand exceeds what kalk can cover. Running both together reduces the total volume of two-part solution you consume. The key is not to over-dose either; test parameters consistently and adjust each component separately.

What happens if I dose too much kalkwasser?

A sudden large dose of kalkwasser causes a rapid pH spike that can stress or kill corals and fish. The practical danger is less about excess calcium (though that matters too) and more about the pH effect. If your pH exceeds 8.5-8.6 after dosing, reduce your ATO dose size and spread doses throughout the day. Kalk should never be added in a large volume all at once.

How do I know if two-part is enough or if I need a calcium reactor?

If you are consistently dosing more than 250-300 ml per day of two-part solution in a 75-gallon tank, you are approaching the cost threshold where a calcium reactor becomes economically competitive. Heavily stocked SPS systems with fast-growing corals often hit this point within a year or two. If two-part keeps up with demand without breaking the bank, there is no reason to switch.

Why does my alkalinity drop faster than my calcium?

Corals consume roughly 1 meq/L of alkalinity for every 20 ppm of calcium - so the two should be depleted in proportion. If alkalinity drops significantly faster than calcium, check whether your two-part ratio is calibrated correctly, and confirm you are using the right concentration for your solution. Low magnesium can also cause alkalinity instability.

Is two-part safe for a tank with fish?

Two-part dosing is safe for fish and invertebrates when dosed correctly - the solutions are dilute enough at proper dose rates that they do not stress livestock. The risk comes from accidental overdosing, which can spike calcium or alkalinity quickly. Use a dosing pump rather than manual dosing whenever possible, and keep a spare batch of RO/DI water on hand for an emergency dilution water change.

If you are just getting started building out your reef chemistry routine, the reef tank water chemistry guide is the best next read - it covers all the major parameters and how they interact before you start dialing in a dosing regimen.

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About the Author

The ReefCraft Guide team writes about saltwater aquarium keeping from hands-on tank experience. From water chemistry to coral placement, our guides reflect what actually works in a home reef setup - not just what the textbook says.