The Complete Guide to Reef Tank Water Parameters

Maintaining proper water chemistry is the foundation of successful reef keeping. Your corals, fish, and invertebrates depend on stable, balanced water parameters to thrive. In this comprehensive guide, we’ll cover the 8 essential parameters every reef keeper should monitor.

Why Water Parameters Matter

Reef aquariums are closed systems that attempt to replicate the chemistry of natural seawater. In the ocean, the sheer volume of water maintains incredibly stable conditions. In our tanks, we need to actively manage these parameters because:

• Corals are sensitive - Even small fluctuations can stress coral and inhibit growth
• Chemistry is interconnected - Parameters like calcium, alkalinity, and magnesium are linked
• Problems compound - Small issues become big problems if left unchecked
• Stability matters more than perfection - Consistent parameters beat “perfect” numbers that fluctuate

The Essential Eight Parameters

1. Calcium (Ca) - Target: 420 ppm

Calcium is the primary building block of coral skeletons. Stony corals actively extract calcium from the water to build their calcium carbonate structures.

Ideal Range: 380-450 ppm

Why It Matters:

• Essential for coral skeleton growth
• Required by coralline algae, clams, and other calcifiers
• Low calcium = slow growth and weak skeletons
• High calcium (>500 ppm) can precipitate with alkalinity

Testing Frequency: Weekly, or more often in heavily stocked SPS tanks

2. Alkalinity (Alk) - Target: 9.0 dKH

Alkalinity measures the water’s buffering capacity—its ability to resist pH changes. It provides the carbonate ions corals need for calcification.

Ideal Range: 7-11 dKH (many SPS keepers prefer 7.5-8.5)

Why It Matters:

• Most consumed parameter in reef tanks
• Directly affects calcification rate
• Buffers pH stability
• Low alk = pH swings and slow growth
• Rapid alk changes are extremely stressful to corals

Testing Frequency: Every 1-3 days for SPS tanks, weekly for mixed reefs

3. Magnesium (Mg) - Target: 1350 ppm

Magnesium prevents calcium and alkalinity from precipitating out of solution. Think of it as the “glue” that holds your chemistry together.

Ideal Range: 1250-1400 ppm

Why It Matters:

• Enables stable calcium/alkalinity relationship
• Low magnesium = difficulty maintaining calc/alk
• Required by some organisms directly
• Generally stable and consumed slowly

Testing Frequency: Weekly to bi-weekly

4. Salinity - Target: 1.026 SG

Salinity measures the total dissolved salts in your water. It affects osmotic balance for all organisms.

Ideal Range: 1.024-1.027 specific gravity (32-35 ppt)

Why It Matters:

• Affects all chemical processes
• Organisms are adapted to specific salinity ranges
• Rapid changes cause osmotic stress
• Evaporation concentrates salinity (top off with fresh water, not saltwater)

Testing Frequency: Daily visual check of ATO, weekly calibrated reading

5. pH - Target: 8.2

pH measures how acidic or alkaline your water is. Reef tanks need to stay alkaline.

Ideal Range: 8.0-8.4

Why It Matters:

• Affects biological processes and calcification
• Influenced by CO2 levels, alkalinity, and bioload
• Natural daily fluctuation is normal (lights on = higher pH)
• Stability matters more than hitting an exact number

Testing Frequency: Continuous monitoring (probe) or weekly testing

6. Temperature - Target: 78°F

Reef organisms are adapted to tropical temperatures. Stability is critical.

Ideal Range: 76-80°F (some keepers run 75-78°F)

Why It Matters:

• Affects metabolism, oxygen levels, and disease resistance
• Temperature swings (>2°F daily) stress organisms
• High temps reduce oxygen and can trigger bleaching
• Consistent temperature supports consistent chemistry

Testing Frequency: Continuous monitoring (heater/chiller controller)

7. Nitrate (NO3) - Target: 5 ppm

Nitrate is the end product of the nitrogen cycle. It’s a nutrient that feeds coral and algae.

Ideal Range: 1-10 ppm (varies by methodology)

Why It Matters:

• Too low = pale, starving coral (especially LPS/softies)
• Too high = algae problems and potential coral stress
• Ultra-low nutrient systems (ULNS) aren’t for everyone
• Balance with phosphate is key

Testing Frequency: Weekly

8. Phosphate (PO4) - Target: 0.05 ppm

Phosphate is another nutrient that affects coral growth and coloration.

Ideal Range: 0.02-0.1 ppm

Why It Matters:

• Too high = inhibits calcification, promotes algae
• Too low = nutrient deficiency, pale coral
• Harder to remove than nitrate
• Ratio with nitrate matters (aim for ~100:1 N:P)

Testing Frequency: Weekly

The Calcium-Alkalinity-Magnesium Relationship

These three parameters are interconnected in a relationship reef keepers call the “ionic balance” or “Cal/Alk/Mag triangle.”

Key principles:

1. Calcium and alkalinity are consumed together during calcification
2. Magnesium must be in range for the relationship to stay stable
3. If one is off, the others will be affected
4. Dose in balance - don’t just boost one parameter

When troubleshooting chemistry issues, always check all three. Low magnesium is often the hidden cause of calc/alk instability.

Testing Tips

• Consistency matters - Test at the same time of day
• Calibrate regularly - Especially pH and salinity probes
• Use quality test kits - Hanna checkers, Salifert, or Red Sea are popular choices
• Track your results - Use ReefTanker to see trends over time
• Test your salt mix - Know what your water change water provides

Maintaining Stability

The secret to reef success isn’t hitting perfect numbers—it’s maintaining stability. Here’s how:

1. Test regularly and track results to spot trends
2. Make small adjustments - Never change parameters rapidly
3. Automate what you can - Dosers, ATO, and controllers help
4. Understand consumption - Know how fast your tank uses calc/alk
5. Water changes help - They replenish trace elements and export waste

Next Steps

Now that you understand the essential parameters, start tracking them consistently. The patterns in your data will reveal how your tank behaves and guide your maintenance decisions.

Ready to start tracking? Try ReefTanker to log your parameters, visualize trends, and master your reef chemistry.