Calcium, Alkalinity, and Magnesium: The Big 3 Explained

If there’s one thing that separates successful reef keepers from frustrated ones, it’s understanding the relationship between calcium, alkalinity, and magnesium. These three parameters—often called “The Big 3” or the “Cal/Alk/Mag triangle”—form the foundation of reef tank chemistry.

Get them right, and your corals will grow, color up, and thrive. Get them wrong, and you’ll fight a never-ending battle against unstable parameters, slow growth, and stressed livestock.

In this guide, we’ll break down exactly how these three parameters work together, why they matter, and how to keep them stable in your reef tank.

Why These Three Parameters Matter Most

Every stony coral in your tank—whether it’s a branching Acropora, a massive brain coral, or the purple coralline algae creeping across your rocks—builds its skeleton from calcium carbonate (CaCO₃). To create this skeleton, corals need:

• Calcium ions (Ca²⁺) — The calcium part of calcium carbonate
• Carbonate ions (CO₃²⁻) — Provided by your alkalinity
• Magnesium — The stabilizer that keeps the whole system working

This process, called calcification, is happening constantly in a healthy reef tank. Every time your corals grow, they’re pulling calcium and alkalinity out of your water. Without replenishment, these levels drop, growth slows, and problems begin.

Understanding Each Parameter

Let’s look at each parameter individually before explaining how they interact.

Calcium: The Building Block

Target Range: 380-450 ppm (ideal: 420 ppm)

Calcium is the primary structural element in coral skeletons. When a coral calcifies, it extracts calcium ions from the surrounding water and combines them with carbonate to form aragonite—a crystalline form of calcium carbonate.

What consumes calcium:

• SPS corals (fastest consumers)
• LPS corals
• Coralline algae
• Tridacna clams
• Tube worms and other calcifying invertebrates

Signs of low calcium:

• Slow or stalled coral growth
• Thin, brittle coral skeletons
• Coralline algae stops spreading
• Clam shells become thin

Signs of high calcium (over 480 ppm):

• White precipitate forming on equipment
• Calcium “snow” in the water column
• Difficulty maintaining alkalinity
• Crusty deposits on heaters and pumps

Alkalinity: The Fuel for Growth

Target Range: 7-11 dKH (ideal: 8-9 dKH)

Alkalinity measures the buffering capacity of your water—specifically, the concentration of carbonate (CO₃²⁻) and bicarbonate (HCO₃⁻) ions. These ions serve two critical functions:

1. Providing carbonate for calcification — Corals need carbonates to build skeletons
2. Buffering pH — Alkalinity prevents rapid pH swings

Alkalinity is the most consumed parameter in a reef tank. While calcium might drop 5-10 ppm per day in a heavily stocked SPS tank, alkalinity can drop 0.5-1.5 dKH in the same period. This is why experienced reef keepers test alkalinity more frequently than any other parameter.

What consumes alkalinity:

• All calcifying organisms (same as calcium)
• Bacterial processes
• pH buffering reactions

Signs of low alkalinity:

• pH instability and large daily swings
• Slow coral growth
• Weak coral skeletons
• Coral tissue recession in severe cases

Signs of high alkalinity (over 12 dKH):

• Coral tissue damage (burnt tips on SPS)
• Precipitation with calcium
• Tissue necrosis if raised too quickly

Critical warning: Rapid alkalinity changes are one of the fastest ways to kill corals. Never change alkalinity by more than 1 dKH per day—and slower is always better. A sudden drop from 9 to 7 dKH can trigger rapid tissue necrosis (RTN) in sensitive SPS corals.

Magnesium: The Unsung Hero

Target Range: 1250-1400 ppm (ideal: 1350 ppm)

Magnesium doesn’t get the attention that calcium and alkalinity do, but it might be the most important of the three. Here’s why: magnesium prevents calcium and alkalinity from precipitating out of solution.

In seawater chemistry, calcium and carbonate ions naturally want to combine and form solid calcium carbonate. Magnesium ions interfere with this process by inserting themselves into the crystal structure, making precipitation much harder. Without adequate magnesium, calcium and alkalinity become unstable and can crash together.

Think of magnesium as the referee that keeps calcium and alkalinity from fighting each other.

Signs of low magnesium:

• Difficulty maintaining stable calcium and alkalinity
• Parameters that “bounce” despite consistent dosing
• White precipitation on equipment
• Calcium and alkalinity that seem linked (when one drops, the other rises)

Signs of high magnesium (over 1500 ppm):

• Inhibited calcification
• Coral stress in extreme cases
• Wasted dosing supplements

How The Big 3 Work Together

Understanding the individual parameters is important, but understanding how they interact is crucial. Here’s what’s happening in your tank:

The Calcification Process

When a coral builds skeleton:

1. The coral extracts calcium ions (Ca²⁺) from the water
2. It combines them with carbonate ions (CO₃²⁻) from your alkalinity
3. This forms calcium carbonate (CaCO₃), which becomes the skeleton
4. Magnesium keeps this process stable by preventing unwanted precipitation

Because calcium and carbonate are consumed together in a fixed ratio, they need to be replenished together. This is why two-part dosing systems work—they add calcium and alkalinity in the same ratio corals consume them.

The Precipitation Problem

Here’s where magnesium becomes critical. Calcium carbonate has a natural tendency to precipitate (fall out of solution as a solid). In your tank, this shows up as:

• White crusty deposits on heaters and pumps
• Cloudy water (“snowstorm”)
• Rapid drops in both calcium and alkalinity

Magnesium prevents this by:

• Interfering with calcium carbonate crystal formation
• Keeping calcium and carbonate ions dissolved and available
• Maintaining the supersaturated state that reef tanks need

The rule of thumb: Magnesium should be approximately 3x your calcium level. If calcium is 420 ppm, magnesium should be around 1260 ppm. This ratio mirrors natural seawater.

The Ionic Balance

Your tank exists in a state of chemical equilibrium. When you change one parameter, you affect the others:

• Raise calcium without raising alkalinity → Calcium may precipitate with existing carbonates, dropping both
• Raise alkalinity without raising calcium → Same problem in reverse
• Low magnesium → Both calcium and alkalinity become unstable
• Proper magnesium → Calcium and alkalinity stay where you put them

This interconnection is why “chasing numbers” by dosing individual elements rarely works. The system needs balance.

Practical Dosing Strategies

Now that you understand the chemistry, let’s talk about how to actually maintain these parameters.

Option 1: Two-Part Dosing

Two-part solutions (like B-Ionic, BRS two-part, or ESV B-Ionic) are the most common method for maintaining calcium and alkalinity. They work by adding calcium and alkalinity in balanced amounts.

Pros:

• Easy to use and adjust
• Precise control over dosing amounts
• Works well with dosing pumps for automation
• No equipment maintenance

Cons:

• Adds sodium chloride over time, requiring water changes
• Can be expensive for high-consumption tanks
• Requires regular testing to dial in amounts

How to use two-part:

1. Test your calcium and alkalinity
2. Calculate your daily consumption (test again in 24-48 hours)
3. Dose equal parts of each solution to match consumption
4. Adjust as needed based on test results

Pro tip: Don’t dose both parts at the same time or in the same location. Space them out by at least an hour, or dose in different areas of the tank. Adding calcium and alkalinity simultaneously in concentrated form can cause localized precipitation.

Option 2: Kalkwasser (Limewater)

Kalkwasser is saturated calcium hydroxide solution, traditionally dripped as top-off water. It provides both calcium and alkalinity while raising pH.

Pros:

• Provides calcium and alkalinity in balanced ratio
• Raises pH (beneficial for tanks with low pH)
• Precipitates phosphate (minor benefit)
• Cost-effective for large tanks

Cons:

• Limited concentration means it can’t keep up with high-demand tanks alone
• Requires careful implementation to avoid pH spikes
• Can be messy to work with
• Not suitable for tanks with already-high pH

Best used: As a supplement to two-part dosing, or as the sole method for lower-demand tanks (LPS and soft coral systems).

Option 3: Calcium Reactor

Calcium reactors dissolve calcium carbonate media (aragonite) using CO₂, releasing calcium and alkalinity into your tank water.

Pros:

• Self-balancing—provides calcium and alkalinity in natural ratios
• Cost-effective for high-consumption tanks long-term
• Minimal daily maintenance once dialed in
• Doesn’t add extra ions to your system

Cons:

• High upfront cost
• Requires CO₂ system
• Can lower pH (the CO₂ effect)
• Takes time to dial in properly
• Equipment to maintain and monitor

Best used: For SPS-dominant tanks with high calcium and alkalinity demand. The break-even point vs. two-part is typically around 80-100 mL/day of two-part consumption.

What About Magnesium?

Magnesium is consumed much more slowly than calcium and alkalinity—often 10-20x slower. Most reef keepers handle magnesium through:

1. Water changes — Quality salt mixes contain magnesium at proper levels. Regular water changes often maintain magnesium without additional dosing.

2. Occasional supplementation — If magnesium drops below 1250 ppm, dose a magnesium supplement (like magnesium chloride/sulfate mix) to bring it back up.

3. Regular testing — Test magnesium weekly. If it’s stable, you can reduce to bi-weekly or monthly testing.

Important: Raise magnesium slowly—no more than 50-100 ppm per day. Rapid magnesium changes can stress corals.

Troubleshooting Common Problems

Problem: Calcium and Alkalinity Won’t Stay Stable

Most likely cause: Low magnesium

Solution:

1. Test magnesium immediately
2. If below 1250 ppm, raise it before doing anything else
3. Raise magnesium by 50-100 ppm per day until you reach 1350 ppm
4. Once magnesium is stable, calcium and alkalinity should stabilize too

Problem: Calcium High, Alkalinity Low (or Vice Versa)

Most likely cause: Imbalanced dosing

Solution:

1. Check your two-part dosing ratio—are you adding equal amounts?
2. Verify your test kits are accurate (test with reference solutions or have LFS verify)
3. Adjust dosing to favor the low parameter until they balance
4. Make changes gradually—don’t try to fix everything in one day

Problem: White Precipitation on Equipment

Most likely cause: Calcium and/or alkalinity too high, or low magnesium

Solution:

1. Test all three parameters immediately
2. Stop all dosing until parameters come down naturally
3. Check magnesium—low magnesium makes precipitation more likely
4. Clean affected equipment
5. Resume dosing at lower levels once parameters normalize

Problem: Parameters Dropping Faster Than Expected

Most likely cause: More coral growth (good!), or something consuming parameters

Solution:

1. Verify with multiple tests that parameters are actually dropping
2. Look for new calcification activity (coral growth, coralline spreading)
3. Check for precipitation on equipment
4. Increase dosing to match new consumption
5. Consider upgrading to a calcium reactor if consumption is very high

Problem: Coral Growth Is Slow Despite Good Numbers

Most likely cause: Instability rather than the actual numbers

Solution:

1. Focus on stability, not hitting “perfect” targets
2. Ensure alkalinity isn’t swinging more than 0.5 dKH between tests
3. Consider whether you’re changing parameters too frequently
4. A stable 8 dKH is better than swinging between 8.5 and 9.5 dKH

Creating Your Testing Schedule

Here’s a practical testing schedule for the Big 3:

SPS-Dominant Tanks

Parameter	Frequency	Notes
Alkalinity	Every 2-3 days	Most critical—swings cause RTN
Calcium	Weekly	More stable than alkalinity
Magnesium	Weekly	Very stable, can reduce to bi-weekly

Mixed Reef Tanks

Parameter	Frequency	Notes
Alkalinity	Weekly	Still the priority
Calcium	Weekly	Test same day as alkalinity
Magnesium	Bi-weekly	Monthly once stable

LPS and Soft Coral Tanks

Parameter	Frequency	Notes
Alkalinity	Weekly to bi-weekly	Lower consumption
Calcium	Bi-weekly	Very low consumption
Magnesium	Monthly	Usually maintained by water changes

Quick Reference: The Big 3 Targets

Parameter	Target	Acceptable Range	NSW Value
Calcium	420 ppm	380-450 ppm	420 ppm
Alkalinity	8-9 dKH	7-11 dKH	7 dKH
Magnesium	1350 ppm	1250-1400 ppm	1280 ppm

Remember: Natural seawater (NSW) values are perfectly valid targets. Many reef keepers run slightly elevated alkalinity for faster growth, but corals evolved in NSW conditions.

Key Takeaways

1. Calcium, alkalinity, and magnesium work together — You can’t manage one without considering the others.

2. Magnesium is the foundation — If your magnesium is low, calcium and alkalinity will never be stable. Always check magnesium first when troubleshooting.

3. Stability trumps perfection — Consistent parameters at 8 dKH are better than parameters that swing between 8.5 and 9.5 dKH.

4. Dose in balance — Use two-part systems, calcium reactors, or kalkwasser to add calcium and alkalinity together.

5. Test regularly and track trends — Single readings mean less than patterns over time. Trending data tells you if your system is stable.

6. Make changes slowly — Never adjust alkalinity by more than 1 dKH per day. Patience prevents disasters.

Start Tracking Your Big 3

Understanding the calcium-alkalinity-magnesium relationship is the foundation of successful reef keeping. But knowledge without data is just theory.

Try ReefTanker to log your parameters, visualize the relationships between your Big 3, and spot trends before they become problems. Our app makes it easy to track consumption rates, set testing reminders, and understand exactly what your tank needs.

Have questions about calcium, alkalinity, or magnesium? Contact us or follow us on social media for more tips and guides.