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Reef Tank Dosing Guide: Two-Part, Kalkwasser & Calcium Reactors

A comprehensive guide to maintaining calcium, alkalinity, and magnesium in your reef aquarium — from knowing when to start dosing, to choosing between two-part, kalkwasser, calcium reactors, and all-in-one solutions. Compiled from Bulk Reef Supply, Randy Holmes-Farley, Reef2Reef, ATI, and the reef keeping community.

18 min read Sources: 12 expert articles

1. Why Reef Tanks Need Dosing

Corals — especially stony corals (SPS and LPS) — are biological factories that consume calcium and carbonate ions from the water column to build their calcium carbonate skeletons [1]. In a closed reef aquarium, these elements are finite. Without replenishment, calcium and alkalinity steadily decline, slowing coral growth and eventually threatening their health [3].

Water changes alone replenish some of these elements, but in tanks with significant coral biomass — particularly SPS-heavy systems — the consumption rate far outpaces what periodic water changes can replace [1] [2]. That’s where dosing comes in: the deliberate addition of calcium, alkalinity, and magnesium supplements to maintain stable levels.

When to Start Dosing

Not every tank needs dosing right away. A good rule of thumb: start dosing when your alkalinity drops 1–2 dKH between weekly water changes [1]. If your parameters stay stable with water changes alone, there’s no need to add complexity. As one experienced reefer puts it, “9 out of 10 tanks don’t need it” in the first year [2]. New tanks with few corals simply don’t consume enough calcium and alkalinity to warrant dosing [2].

Key Principle: Don’t dose until your tank tells you it needs it. Test your water weekly — when alkalinity consistently drops between water changes, that’s your signal to begin. Starting too early adds unnecessary complexity and risk. [2]

2. The Big Three Parameters

Calcium, alkalinity, and magnesium form an interconnected triad. You cannot manage one without understanding how it affects the other two [11]. Here are the target ranges and critical relationships:

Parameter Target Range Max Daily Change Source
Calcium (Ca) 400–420 ppm 50–100 ppm [1] [9]
Alkalinity (dKH) 7–9 dKH 1–1.4 dKH [1] [9]
Magnesium (Mg) ~1300 ppm 100 ppm [1] [11]

The Ca/Alk/Mg Relationship

Calcium and alkalinity exist in a delicate balance. When calcium exceeds 500 ppm, alkalinity begins to fall as calcium carbonate precipitates out of solution [11] [9]. Conversely, when alkalinity climbs above 10–12 dKH, calcium precipitates and drops [11]. This is why you should never chase one parameter without watching the other.

Magnesium is the stabilizer. At proper levels (~1300 ppm), magnesium inhibits spontaneous calcium carbonate precipitation by interfering with crystal growth [11]. If magnesium is low, it becomes nearly impossible to maintain calcium and alkalinity at target levels — they will crash together [11].

Critical: Always check magnesium first. If Mg is below 1200 ppm, correct it before adjusting calcium or alkalinity. Low magnesium is the hidden cause of most “unexplained” Ca/Alk crashes. [11]

3. Five Dosing Methods Compared

There are multiple ways to replenish calcium and alkalinity. Each has trade-offs in cost, complexity, and suitability for different tank sizes and coral loads [10].

Method Setup Cost Ongoing Cost Complexity Best For
Two-Part $30–80 Medium Low–Medium Mixed Reef
Kalkwasser $10–50 Very Low Low Nano Mixed
Calcium Reactor $300–800+ Very Low High SPS Heavy
All-in-One $0 Medium–High Very Low Nano LPS
Balling (DIY) $20–50 Low Medium SPS Value

a. Two-Part Dosing (BRS Pharma, ESV B-Ionic)

The most popular method for home reef tanks. Two separate solutions — one for alkalinity (soda ash/sodium carbonate) and one for calcium (calcium chloride) — are dosed in balanced amounts [1] [8].

Alk dose: Soda ash 0.3 mL/gal = +0.5 dKH [8]
Ca dose: CaCl2 1 mL/gal = +10 ppm Ca [8]
Never dose both solutions simultaneously. Concentrated calcium and alkalinity mixed together will precipitate as calcium carbonate, wasting your chemicals and potentially causing a milky “snowstorm” in your tank. Wait 20–30 minutes between dosing each part. [9] [8]

b. Kalkwasser (Limewater)

One of the oldest and cheapest dosing methods. Calcium hydroxide (Ca(OH)2) is dissolved in freshwater at ½ to 2 teaspoons per gallon of freshwater, producing a saturated solution with a pH above 12 [5]. When dripped into the tank, it simultaneously replenishes both calcium and alkalinity [5].

Provides: Ca + Alk
Bonus: Precipitates phosphate [5]
Best time: Night (offsets pH drop)
Limitation: Capped by evap rate [10]

Kalkwasser is limited by your tank’s evaporation rate — you can only add as much limewater as the tank evaporates [10]. Best dosed at night to counteract the natural overnight pH drop [5]. The cheapest ongoing cost of any dosing method [10].

c. Calcium Reactor

A calcium reactor uses CO2 gas to lower pH inside a chamber filled with aragonite media, dissolving the media and releasing calcium and carbonate into the tank water [7]. The upfront cost is significant — $300–800+ for the reactor, CO2 tank, regulator, and solenoid [7].

Setup cost: $300–800+ [7]
Best for: 120+ gal SPS-heavy [7]
Ongoing cost: Lowest
Media: Aragonite / coral skeleton

For SPS-dominant tanks above 120 gallons, a calcium reactor provides the lowest ongoing cost and most stable supplementation [7]. The trade-off is complexity: CO2 management, effluent tuning, and the risk of pH suppression from excess CO2 [10].

d. All-in-One (Tropic Marin All-For-Reef)

A single-bottle solution that provides calcium, alkalinity, magnesium, and trace elements in one dose [12]. The simplest method available — ideal for beginners and nano tanks.

Start dose: 5 mL per 26 gal daily [12]
Increase: +2.5 mL/week [12]
Max dose: 25 mL daily [12]
Format Cost per Gallon Source
Liquid (pre-mixed) ~$140/gal [12]
DIY concentrate ~$45.50/gal [12]
Powder (mix yourself) ~$28.50/gal [12]

e. Balling Method (Holmes-Farley DIY Two-Part)

A cost-effective DIY approach using pharmacy-grade chemicals. Randy Holmes-Farley’s recipes are the gold standard for home-mixed two-part solutions [4].

Recipe #1 — Normal pH Tanks (≤8.3)

Calcium Solution

500 g CaCl2·2H2O (Dow Flake) per gallon of RO/DI water [4]

Alkalinity Solution

594 g baked baking soda (soda ash) per gallon of RO/DI water [4]

Recipe #2 — High pH Tanks (>8.3)

Use half the concentrations of Recipe #1, and substitute raw (unbaked) baking soda for soda ash. The sodium bicarbonate produces less pH increase per dose, preventing pH spikes in already-alkaline systems [4].

Baking soda vs. soda ash: Bake baking soda at 300°F (150°C) for 1 hour to convert sodium bicarbonate into soda ash (sodium carbonate). Soda ash raises pH more per dose. Use raw baking soda if your pH is already above 8.3. [4]

4. Method Selection Flowchart

Choosing the right dosing method depends on your tank size, coral type, and willingness to manage equipment [10] [1]:

FOWLR / Soft Coral → Water changes only

Soft corals and fish-only tanks rarely consume enough Ca/Alk to warrant dosing. Regular water changes with a quality salt mix are sufficient [2].

Nano <30 gal → All-For-Reef or Kalkwasser

Small tanks have low consumption rates. A simple all-in-one solution keeps things manageable without multiple dosing pumps [12].

Mixed Reef (Medium) → Two-Part + Dosing Pump

The sweet spot for most hobbyists. Automated two-part dosing with a multi-head pump provides precise control at moderate cost [1] [3].

SPS-Dominant >120 gal → Calcium Reactor + Kalkwasser

Heavy SPS tanks consume enormous amounts of Ca/Alk. A calcium reactor handles the bulk supplementation while kalkwasser in the ATO tops up and stabilizes pH [7] [10].

5. Dosing Pump Schedules

How often you dose matters as much as how much. Frequent small doses maintain more stable parameters than one large daily dump [9] [3].

Schedule Doses/Day Stability Best For
Minimum 1×/day Acceptable Low coral load, soft coral
Standard 4–6×/day Good Mixed reef, LPS
Optimal 12–24×/day Excellent SPS-dominant

Scheduling Rules

  • Never dose alkalinity and calcium at the same time — alternate them with at least 20–30 minutes apart [9] [8]
  • Calcium during the day — corals calcify faster under light [9]
  • Alkalinity at night — helps buffer the natural overnight pH drop [9]
  • For 12–24 dose schedules, alternate between Ca and Alk every 30–60 minutes throughout the day [9]
Pro tip: Most modern dosing pumps (Red Sea ReefDose, GHL, Kamoer) let you program multiple daily doses with custom intervals. Set up alternating Ca/Alk schedules and let the pump handle the timing. [3]

6. How to Calculate Your Dose

Every tank consumes calcium and alkalinity at different rates depending on coral biomass, growth rate, and water chemistry. Here’s how to determine your specific consumption [1]:

1

Test Day 1

After a water change, test and record calcium (ppm), alkalinity (dKH), and magnesium (ppm). Note the exact time [1].

2

Test Day 2

At the same time the next day (24 hours later), test the same parameters again [1].

3

Calculate Consumption

Daily consumption = Day 1 reading − Day 2 reading. For example, if alkalinity was 8.5 dKH on Day 1 and 8.0 dKH on Day 2, your daily consumption is 0.5 dKH [1].

4

Calculate Dose Volume

Using BRS Pharma Soda Ash as an example: if your tank is 50 gallons and consumes 0.5 dKH/day, you need 0.3 mL/gal × 50 gal = 15 mL of soda ash solution per day [8]. Divide this across your dosing schedule (e.g., 3.75 mL × 4 doses).

Online Dosing Calculators

Rather than doing the math by hand, these free calculators will tell you exactly how much to dose [6]:

BRS Calculator Diesyst.com Seachem ReefApp Pod Your Reef ReefStable New Wave Aquaria [6]

7. Dosing Pump Recommendations

A reliable dosing pump automates the tedious work of manual dosing and dramatically improves parameter stability [3].

Red Sea ReefDose

  • 2-head or 4-head models
  • WiFi connected, app-controlled
  • Auto-calibration, high accuracy
  • Premium pick for ease of use

GHL Doser 2.1

  • 4 independent heads
  • ProfiLux integration
  • Industrial-grade precision
  • Best for controller ecosystems

Kamoer X1 Pro

  • WiFi, app-controlled
  • Stackable modular design
  • Good accuracy, mid-range price
  • Popular budget-to-mid option

Jebao DP-4

  • 4-head peristaltic pump
  • Very affordable ($40–60)
  • Manual programming only
  • Solid budget pick, widely used
Calibration matters: Regardless of brand, calibrate your dosing pump monthly. Peristaltic tubing stretches over time, changing flow rates. A 5% drift in dosing accuracy can cause measurable parameter swings over weeks [3].

8. Common Dosing Mistakes

Overdosing

Problem: Adding too much calcium or alkalinity, pushing parameters above safe ranges. When Ca exceeds 500 ppm or Alk exceeds 12 dKH, precipitation cascades begin [11] [9].

Fix: Always start with a calculated dose based on measured consumption. Increase gradually. More is not better — stability at 8 dKH beats a spike to 11 dKH [9].

Dosing Calcium and Alkalinity Simultaneously

Problem: Mixing concentrated Ca and Alk solutions causes immediate calcium carbonate precipitation. In the tank, simultaneous dosing creates localized super-saturation [9].

Fix: Space doses at least 20–30 minutes apart. If using a multi-head pump, stagger the schedule so Ca and Alk never run at the same time [9] [8].

Not Testing Regularly

Problem: Coral consumption rates change as corals grow. A dose that was perfect 3 months ago may be insufficient (or excessive) today [1] [3].

Fix: Test alkalinity at least 2–3 times per week. Calcium and magnesium can be tested weekly. Adjust doses as consumption changes [1].

Chasing Numbers

Problem: Making large corrections to hit “perfect” numbers. Rapid parameter swings stress corals more than slightly off-target but stable levels [9].

Fix: Make corrections gradually — no more than 1 dKH Alk change or 50 ppm Ca change per day [9]. A stable 7.5 dKH is far better for corals than swinging between 8 and 9 dKH [9].

Starting Before the Tank Needs It

Problem: Dosing a new tank with few corals leads to rising, uncontrolled calcium and alkalinity levels [2].

Fix: Only start dosing when testing shows consistent decline between water changes. Let the tank tell you when it’s ready [2] [1].

9. Frequently Asked Questions

Do I need to dose my reef tank?

Not necessarily. If your alkalinity and calcium remain stable with regular water changes alone, dosing is unnecessary [2]. Most new tanks with few corals don’t need it. Start dosing only when you measure a consistent decline of 1–2 dKH between weekly water changes [1].

Can I dose alkalinity and calcium at the same time?

No. Concentrated calcium and alkalinity solutions will react and precipitate as calcium carbonate if mixed or dosed simultaneously [9]. Always space them at least 20–30 minutes apart, or alternate throughout the day [8].

Which is better: two-part or a calcium reactor?

It depends on tank size and coral load. Two-part is simpler, cheaper to start, and works well for tanks under 120 gallons [1]. A calcium reactor has higher upfront cost ($300–800+) but lower ongoing cost and provides more stable supplementation for large, SPS-heavy systems [7]. Many experienced reefers combine a calcium reactor with kalkwasser in the ATO for the best of both worlds [10].

How often should I test when dosing?

When first starting to dose, test alkalinity daily until you’ve dialed in the correct amount. Once stable, test 2–3 times per week for alkalinity and weekly for calcium and magnesium [1] [3]. Coral consumption rates change over time as corals grow, so periodic recalibration is essential.

What if my calcium is high but alkalinity is low?

This is a common imbalance. First check magnesium — if it’s below 1200 ppm, raise it before adjusting anything else [11]. Then dose only alkalinity (no calcium) until the ratio normalizes. Never try to fix both at once by overdosing one side [9]. If calcium is above 500 ppm, stop calcium dosing entirely and let the tank consume it naturally [11].

Is kalkwasser safe for my tank?

Yes, when used correctly. Kalkwasser has a pH above 12, so it must be dripped slowly — never dumped directly into the tank [5]. It’s limited by your evaporation rate, which acts as a natural safety cap [10]. It’s one of the safest and cheapest dosing methods, with the added benefit of precipitating phosphate [5].

References

Every factual claim in this guide is cited to its original source. Click any [n] in the text above to jump here.

  1. Reef Tank Resource — “Reef Tank Dosing Guide”
  2. The Beginners Reef — “Do I Need to Dose My Reef Tank?”
  3. ATI North America — “A Beginner’s Guide to Reef Dosing”
  4. Randy Holmes-Farley — “DIY Two-Part Recipe” (Reefkeeping Magazine, 2006)
  5. Saltwater Aquarium Blog — “Kalkwasser Guide”
  6. New Wave Aquaria — “Dosing Calculator”
  7. Bulk Reef Supply — “Calcium Reactor vs. Two-Part Dosing”
  8. Bulk Reef Supply — “BRS Pharma Soda Ash Instructions”
  9. Reef2Reef — “Dosing 101 Rules for Your Reef Aquarium”
  10. Reef2Reef — “Pros and Cons of Alkalinity and Calcium Dosing Methods”
  11. ReefStable — “Dosing Calcium, Alkalinity, and Magnesium”
  12. Bulk Reef Supply — “Tropic Marin All-For-Reef”

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