The Complete Guide to Dosing in a Reef Tank: Alkalinity, Calcium & Magnesium

If you've been reefing for more than a few months, you've probably heard the phrase "stable parameters" more times than you can count. And for good reason — stability is the single biggest predictor of coral health and growth.

But stability doesn't happen by accident. As your corals grow, they consume alkalinity, calcium, and magnesium from the water. If you're not replacing what's being used, your parameters will drift — and drifting parameters mean stressed, bleached, or dying coral.

That's where dosing comes in.

Why Corals Deplete Your Water Chemistry

Corals build their skeletons from calcium carbonate (CaCO₃). To do this, they pull three key elements from the water column:

• Alkalinity (dKH) — the primary building block of coral skeleton; also buffers pH swings
• Calcium (Ca²⁺) — works alongside alkalinity to form the calcium carbonate skeleton
• Magnesium (Mg²⁺) — keeps calcium and alkalinity in solution and prevents precipitation; also used directly by coral tissue

These three elements are chemically linked. You can't maintain one without the others — if magnesium is too low, calcium and alkalinity will precipitate out of solution no matter how much you dose. Think of magnesium as the glue that holds the system together.

In a lightly stocked tank with regular water changes, natural replenishment may be enough. But in any established reef with growing coral, you'll need to actively dose to keep up with consumption.

Target Parameters

Before you start dosing, know your targets:

Important: Never chase a specific number at the expense of stability. A tank running consistently at 8 dKH is far healthier than one swinging between 7 and 10. Slow, gradual adjustments always win.

Testing Before You Dose

Dosing without testing is guesswork — and guesswork in a reef tank is expensive. Before starting any dosing regimen:

1. Test your current parameters — alkalinity, calcium, and magnesium as a baseline
2. Test at the same time each day — alkalinity fluctuates with pH and lighting cycles; consistency in timing gives you consistent data
3. Use quality test kits — Salifert, Hanna checkers, or ICP testing for the most accurate results
4. Track your results — a simple spreadsheet or reef journal helps you spot trends before they become problems

Once you know your baseline and how fast your tank consumes each element, you can calculate your daily dosing requirements.

Dosing Methods: Which Is Right for You?

Manual Dosing (Two-Part)

Two-part dosing uses two separate liquid solutions — Part A (alkalinity) and Part B (calcium) — dosed manually or via a simple pump. Magnesium is typically supplemented separately as needed.

Pros:

• Low upfront cost
• Simple to understand and adjust
• No equipment to maintain

Cons:

• Requires daily consistency — miss a dose and parameters drift
• Time-consuming for larger tanks with high consumption
• Harder to dose in small, precise increments

Best for: Tanks under ~300L, lower coral density, or reefers just starting to dose.

Auto Dosing Pumps

An auto doser is a peristaltic pump that delivers precise, programmable doses of two-part solution at set intervals throughout the day. Instead of one large daily dose, it delivers small amounts every few hours — keeping parameters far more stable.

Pros:

• Consistent, automated delivery — no missed doses
• Smaller, more frequent doses = less parameter swing
• Frees up daily maintenance time
• Scalable as your tank grows

Cons:

• Higher upfront cost
• Requires calibration and occasional maintenance
• Still needs regular testing to confirm accuracy

Best for: Any established reef, SPS-dominant tanks, or anyone who values consistency over convenience.

Calcium Reactor

A calcium reactor uses CO₂ to dissolve calcium carbonate media (coral skeleton or aragonite), releasing calcium and alkalinity back into the water in a natural, balanced ratio.

Pros:

• Extremely cost-effective at scale — media is cheap
• Delivers calcium and alkalinity in a naturally balanced ratio
• Ideal for high-demand SPS tanks

Cons:

• Higher complexity — requires CO₂ cylinder, regulator, and pH monitoring
• Takes time to dial in correctly
• pH can drop if not managed carefully

Best for: Large, mature SPS reefs with high calcium/alkalinity demand.

Shop Calcium Reactors →

Kalkwasser (Limewater)

Kalkwasser (calcium hydroxide dissolved in RODI water) is dosed via your auto top-off system, replenishing evaporated water while simultaneously supplementing calcium and alkalinity. It also raises pH and precipitates phosphate.

Pros:

• Raises pH — great for tanks with low pH issues
• Helps control phosphate
• Simple to implement alongside an ATO

Cons:

• Limited capacity — can only dose as fast as your tank evaporates
• Can cause pH spikes if overdosed
• Not sufficient alone for high-demand tanks

Best for: Supplementary dosing alongside two-part or a calcium reactor.

How to Calculate Your Dosing Requirements

Here's a simple method to work out how much to dose:

1. Test your alkalinity at the same time on two consecutive days (without dosing)
2. Calculate the daily drop — e.g., if it drops from 8.5 to 8.0 dKH, your tank consumes 0.5 dKH/day
3. Use a dosing calculator (BRS, Reef Chemistry, or similar) to convert dKH consumption into ml of two-part solution per day
4. Split into multiple doses — divide your daily total across 4–6 doses for maximum stability
5. Retest after 1 week and adjust as needed

Calcium and magnesium consumption can be calculated the same way. As a general rule, calcium and alkalinity are consumed in roughly equal molar amounts — if your alkalinity consumption is high, your calcium demand will be proportionally high too.

Starting a Dosing Regimen: Step by Step

1. Test all three parameters and record your baseline
2. Correct any deficiencies slowly — raise parameters by no more than 1 dKH alkalinity or 10–20 ppm calcium per day to avoid precipitation or coral stress
3. Start dosing at a conservative rate — it's easier to increase than to recover from an overdose
4. Test every 2–3 days for the first two weeks to confirm your dose is hitting the target
5. Settle into a weekly testing rhythm once parameters are stable

Common Dosing Mistakes

• Dosing without testing — you can't know if you're hitting the mark without data
• Raising parameters too quickly — sudden swings are more damaging than low-but-stable levels
• Ignoring magnesium — low Mg causes calcium and alkalinity to precipitate, making your dosing ineffective
• Dosing alkalinity and calcium together — they can react and precipitate if mixed; always dose separately with time between
• Not recalibrating your doser — peristaltic pumps drift over time; calibrate monthly

Choosing Your Dosing Products

Quality matters when it comes to two-part solutions. Cheap or poorly formulated products can introduce unwanted elements or inconsistent concentrations. Look for products that are:

• Specifically formulated for reef aquariums
• Free from unnecessary additives or fillers
• Clearly labelled with concentration (so you can calculate doses accurately)

Shop Coral Essentials Dosing Products →

The Bottom Line

Dosing is what separates a tank that survives from one that thrives. Once you understand what your tank consumes and build a consistent routine around replacing it, you'll find that stable parameters become the norm — not the exception.

Start simple, test regularly, and scale your method as your tank grows. Your corals will reward you for it.

Shop Dosing Products & Reactors →

Related reading:

• Reef Tank Stability vs. Chasing Numbers — What Matters Most?
• What Is ICP Testing & Why Your Reef Needs It
• Stop Chasing Numbers – Focus on Stability Instead