How to Dose Your Reef Aquarium Properly for Stable Coral Health
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How to Dose Your Reef Aquarium Properly for Stable Coral Health
By Dr Duncan Houston
Dosing a reef aquarium is one of the most misunderstood parts of marine fishkeeping. Most reef tank problems are not caused by lack of equipment, but by unstable chemistry driven by incorrect dosing.
Corals rely on calcium, alkalinity, and magnesium to build skeletons, regulate internal balance, and maintain growth. When these parameters fluctuate too quickly or are dosed incorrectly, the result is stress, poor growth, tissue loss, or full system instability.
In practice, many reef failures are not due to lack of care. They are due to chasing numbers, reacting too quickly, or not understanding how consumption actually works in a living system.
This guide breaks down reef dosing properly, from baseline testing to advanced system control, so you can build a stable, predictable, and resilient reef.
Quick Answer
Reef dosing should always be based on measured consumption, not guesswork. Test calcium, alkalinity, and magnesium, pause dosing to measure how much your tank uses, then replace that amount gradually with daily dosing. Stability matters more than exact numbers, and rapid changes are one of the most common causes of coral stress and loss.
What Are You Actually Dosing?
Corals and calcifying organisms depend on three core parameters:
Calcium (Ca)
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Target: ~400 to 450 ppm
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Role: skeletal growth
Low calcium slows growth. Excessively high calcium can contribute to instability and precipitation.
Alkalinity (dKH)
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Target: ~7 to 9 dKH (system dependent)
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Role: pH buffering and carbonate availability
This is the most important and most sensitive parameter.
If something goes wrong in a reef tank, alkalinity is usually the first place to look.
Magnesium (Mg)
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Target: ~1250 to 1350 ppm
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Role: stabilises calcium and alkalinity
Low magnesium makes it difficult to maintain both calcium and alkalinity.
Why Stability Matters More Than Perfect Numbers
One of the biggest mistakes in reef keeping is chasing exact numbers.
In reality:
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Stable parameters are far more important than exact targets
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Corals tolerate slightly imperfect levels better than rapid swings
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Most coral stress events are caused by instability, not deviation
A stable 7.5 dKH is safer than a fluctuating 8.5 dKH.
Step 1: Test Your Parameters Properly
Before dosing anything, establish a baseline.
Test:
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Calcium
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Alkalinity
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Magnesium
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Salinity
If a result looks unusual, repeat it.
Clinical insight
If test results do not match how your corals look, question the test before changing the tank.
Step 2: Correct Major Imbalances Safely
If levels are too low
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Calculate the required correction
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Raise gradually over several days
If levels are too high
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Stop dosing
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Allow natural consumption
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Use controlled water changes if needed
Critical rule
Never make large corrections in a single dose.
This is one of the most common causes of coral stress, bleaching, and tissue damage.
Step 3: Measure Real Consumption
Pause dosing for 5 to 7 days.
Continue normal feeding and lighting.
After this period:
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Re-test calcium, alkalinity, and magnesium
The difference shows your tank’s actual consumption.
Without this step, dosing is guesswork.
Step 4: Understand Consumption Patterns
Consumption is not static.
It changes based on:
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Coral type
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Coral growth stage
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Tank maturity
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Lighting intensity
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Nutrient levels
Real-world patterns
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New tanks consume very little
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SPS-dominated tanks consume rapidly
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Adding coral increases demand within days
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Growth phases can cause sudden increases in consumption
This is why dosing must be adjusted over time.
Step 5: Calculate and Apply Daily Dosing
Once consumption is known:
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Calculate total weekly deficit
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Divide into daily doses
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Dose gradually
Important principle
Never add the full correction at once.
Spread dosing across the day where possible.
Step 6: Dosing Methods Compared
2-part systems
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Calcium and alkalinity separate
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Simple and widely used
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Requires manual balance
3-part systems
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Includes magnesium
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More complete control
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Slightly more complex
Kalkwasser
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Raises calcium and alkalinity together
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Helps stabilise pH
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Risk of overdose if misused
Calcium reactors
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Advanced systems for high-demand tanks
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Very stable when tuned properly
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Higher complexity and cost
Each method has trade-offs between simplicity, control, and risk.
How Alkalinity Links to pH Stability
Alkalinity directly influences pH stability.
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Low alkalinity leads to unstable pH
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High alkalinity combined with heavy dosing can cause pH spikes
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Dosing timing can influence daily pH swings
Practical tip
Many systems benefit from spreading dosing across the day or dosing during periods of natural pH drop.
Precipitation: A Common Hidden Problem
Calcium and alkalinity can bind and fall out of solution.
This is called precipitation.
Signs include:
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Cloudy water
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White residue on pumps or heaters
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Falling calcium despite dosing
Causes:
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Overdosing
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Imbalance between parameters
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High pH combined with high dosing
This reduces available calcium and destabilises the system.
How Serious Are Dosing Errors?
Low risk
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Slight underdosing
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Slow parameter drift
Moderate risk
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Inconsistent dosing
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Mild fluctuations
High risk
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Rapid alkalinity swings
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Large corrections
Critical
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Severe overdose
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Precipitation events
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Sudden parameter spikes
This is where coral loss occurs.
What It Looks Like When Dosing Goes Wrong
Alkalinity swing
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Burnt or white coral tips
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Rapid tissue loss
Underdosing
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Slow growth
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Dull colour
Overdosing
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Cloudy water
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Precipitation on equipment
Instability
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Closed polyps
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Unpredictable coral behaviour
Visual clues often appear before test results confirm the issue.
When Is This an Emergency?
Act immediately if you see:
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Rapid coral tissue loss
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Sudden bleaching
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Multiple colonies closing simultaneously
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Cloudy water from precipitation
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Major alkalinity spike or crash
These indicate serious chemical instability.
What Should You Do If Something Goes Wrong?
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Stop dosing immediately
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Re-test all parameters
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Confirm results
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Correct slowly
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Stabilise before resuming dosing
Doing less is often safer than reacting aggressively.
Common Mistakes
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Dosing without testing
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Chasing perfect numbers
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Large, rapid corrections
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Ignoring magnesium
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Using inaccurate test kits
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Not adjusting for growth
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Mixing supplements incorrectly
Safety Rules for Dosing
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Never mix supplements together
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Dose in high-flow areas
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Label all containers clearly
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Keep supplements separate
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Follow dosing instructions carefully
Water Changes and Dosing
Water changes can:
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Reset parameters
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Reduce imbalances
However:
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Low-demand tanks may rely on water changes alone
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High-demand reef systems still require dosing
Water changes are not a replacement for proper dosing in most coral-heavy tanks.
How to Prevent Dosing Problems
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Test consistently
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Track trends over time
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Adjust slowly
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Maintain stable routines
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Reassess after adding coral
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Focus on long-term stability
Trend-based thinking is key. Single readings are less important than patterns.
FAQ
How often should I test parameters?
At least weekly. Alkalinity may require more frequent testing in high-demand tanks.
Which parameter matters most?
Alkalinity. It changes fastest and causes the most visible issues.
Why did my coral react badly after dosing?
Rapid parameter change is the most common cause.
Do all tanks consume the same amount?
No. Every system is different.
What happens if I overdose alkalinity?
pH spikes, tissue damage, precipitation, and coral stress.
What happens if alkalinity drops?
Reduced growth, tissue recession, and increased disease susceptibility.
Do I need a dosing pump?
Not essential, but it improves consistency significantly.
Why is magnesium important?
It stabilises calcium and alkalinity and prevents precipitation.
Can water changes replace dosing?
Only in low-demand systems.
Should I adjust dosing after every test?
No. Look for trends.
What is the most common mistake?
Guessing dosing instead of measuring consumption.
Final Thoughts
Reef dosing is not about adding supplements. It is about understanding your system, measuring real consumption, and maintaining stability over time.
The best reef tanks are not the ones with perfect numbers. They are the ones with consistent, predictable chemistry.
If you approach dosing like a system rather than a reaction, your corals will grow, stabilise, and become far more resilient.
If you are unsure how to interpret your results or your coral is not responding as expected, ASK A VET™ can help guide your dosing strategy based on your system, livestock, and trends.