Water Parameters Guide - Ideal Ranges for Reef Tanks

Complete Water Parameters Guide for Reef Tanks

Maintaining stable water parameters is the foundation of successful reef keeping. This comprehensive guide covers every parameter you need to monitor, ideal ranges, testing schedules, and how to correct imbalances.

Why Water Parameters Matter

Corals are incredibly sensitive to water chemistry. In the ocean, parameters remain remarkably stable. In our closed reef systems, we must replicate that stability through regular testing and maintenance. Small fluctuations are normal, but consistency is more important than hitting exact numbers.

Essential Parameters

Temperature

Ideal Range: 76-78°F (24-26°C)
Test Frequency: Daily (continuous monitoring recommended)

Why It Matters

  • Affects coral metabolism and growth
  • Influences oxygen levels in water
  • Impacts bacterial processes
  • Stability is critical - fluctuations stress corals

How to Maintain

  • Use a quality heater with thermostat
  • Consider a backup heater
  • Use a chiller if room temperature exceeds 80°F
  • Keep tank away from windows and heat sources
  • Monitor with digital thermometer

Troubleshooting

  • Too high: Increase ventilation, use fans, reduce lighting period, consider chiller
  • Too low: Check heater function, increase heater wattage if needed
  • Fluctuating: Check heater placement, ensure proper water circulation

Salinity (Specific Gravity)

Ideal Range: 1.025-1.026 SG (35 ppt or 35,000 ppm)
Test Frequency: Daily

Why It Matters

  • Affects osmotic balance in coral tissue
  • Influences nutrient uptake
  • Impacts coral coloration
  • Critical for invertebrate health

How to Maintain

  • Use a refractometer (not hydrometers - they're inaccurate)
  • Calibrate refractometer regularly
  • Top off evaporation daily with RODI water
  • Test at consistent temperature (77°F)
  • Use quality salt mix

Troubleshooting

  • Too high: Add small amounts of RODI water, test frequently
  • Too low: Add small amounts of saltwater, test frequently
  • Fluctuating: Top off evaporation more frequently, consider auto top-off system

pH (Acidity/Alkalinity)

Ideal Range: 8.1-8.4
Test Frequency: Weekly (same time of day)

Why It Matters

  • Affects coral calcification
  • Influences nutrient availability
  • Impacts biological processes
  • Natural daily fluctuation is normal

How to Maintain

  • Maintain proper alkalinity (buffers pH)
  • Ensure adequate gas exchange (surface agitation)
  • Run protein skimmer
  • Maintain calcium reactor if using one
  • Consider refugium with reverse photoperiod

Troubleshooting

  • Too low: Increase aeration, check alkalinity, add pH buffer if needed
  • Too high: Rare - check if dosing too much buffer
  • Fluctuating: Normal daily swing of 0.2-0.3 is okay; larger swings indicate CO2 issues

Alkalinity (dKH or Carbonate Hardness)

Ideal Range: 8-11 dKH (2.8-3.9 meq/L)
Test Frequency: 2-3 times per week (daily for SPS-dominant tanks)

Why It Matters

  • Critical for coral skeleton building
  • Buffers pH swings
  • Most important parameter for coral growth
  • Consumed rapidly by corals

How to Maintain

  • Test regularly - consumption varies by coral load
  • Dose 2-part solution (calcium + alkalinity)
  • Use calcium reactor for heavy SPS tanks
  • Kalkwasser (limewater) for top-off
  • Maintain consistency - don't chase numbers

Troubleshooting

  • Too low: Dose alkalinity supplement, increase dosing schedule
  • Too high: Reduce dosing, perform water change
  • Dropping rapidly: Increase dosing frequency, check coral load

Calcium (Ca)

Ideal Range: 400-450 ppm
Test Frequency: Weekly

Why It Matters

  • Essential for coral skeleton formation
  • Works with alkalinity for calcification
  • Consumed by corals, coralline algae, clams
  • Must be balanced with alkalinity and magnesium

How to Maintain

  • Dose 2-part solution (balanced with alkalinity)
  • Use calcium reactor
  • Kalkwasser dosing
  • Regular water changes replenish calcium

Troubleshooting

  • Too low: Dose calcium supplement, check magnesium levels
  • Too high: Reduce dosing, perform water change
  • Won't rise: Check magnesium - low mag prevents calcium uptake

Magnesium (Mg)

Ideal Range: 1250-1350 ppm
Test Frequency: Bi-weekly to monthly

Why It Matters

  • Prevents calcium and alkalinity from precipitating
  • Allows proper calcium/alkalinity balance
  • Consumed slowly but steadily
  • Often overlooked but critical

How to Maintain

  • Dose magnesium supplement as needed
  • Included in quality salt mixes
  • Test when calcium/alk won't stabilize
  • Replenished through water changes

Troubleshooting

  • Too low: Dose magnesium supplement over several days
  • Too high: Rare - reduce dosing, perform water changes

Nitrate (NO3)

Ideal Range: 5-10 ppm (some corals tolerate 10-20 ppm)
Test Frequency: Weekly

Why It Matters

  • End product of nitrogen cycle
  • Some level is beneficial for coral coloration
  • Ultra-low (ULNS) can stress corals
  • High levels promote algae growth

How to Maintain

  • Regular water changes
  • Protein skimming
  • Refugium with macroalgae
  • Don't overfeed
  • Maintain adequate clean-up crew

Troubleshooting

  • Too high: Increase water changes, reduce feeding, add refugium, check filtration
  • Too low (under 1 ppm): May need to feed more, some corals need nutrients

Phosphate (PO4)

Ideal Range: 0.03-0.1 ppm
Test Frequency: Weekly

Why It Matters

  • Nutrient for coral zooxanthellae
  • Some level beneficial for coloration
  • High levels promote algae, inhibit calcification
  • Ultra-low can cause coral stress

How to Maintain

  • Regular water changes
  • Use RODI water (check TDS)
  • GFO (granular ferric oxide) reactor if needed
  • Protein skimming
  • Refugium with macroalgae

Troubleshooting

  • Too high: Water changes, reduce feeding, use phosphate remover, check source water
  • Too low: May need to feed more, reduce GFO use

The Calcium-Alkalinity-Magnesium Relationship

These three parameters work together and must be balanced:

  • Calcium and Alkalinity: Consumed together by corals for skeleton building
  • Magnesium: Prevents calcium and alkalinity from precipitating out of solution
  • Ratio: Maintain Mg at roughly 3x calcium level (e.g., 420 Ca = 1260 Mg)

Important: If calcium or alkalinity won't rise despite dosing, always check magnesium first!

Testing Schedule Summary

Daily

  • Temperature (continuous monitoring ideal)
  • Salinity

2-3 Times Per Week

  • Alkalinity (especially for SPS tanks)

Weekly

  • Calcium
  • Nitrate
  • Phosphate
  • pH

Bi-Weekly to Monthly

  • Magnesium

Testing Tips

  • Test at same time: Parameters fluctuate daily - consistency matters
  • Follow instructions: Each test kit has specific procedures
  • Check expiration dates: Old reagents give false readings
  • Use quality test kits: Salifert, Red Sea, Hanna are reliable brands
  • Keep a log: Track trends over time
  • Calibrate equipment: Refractometers, pH meters need regular calibration

Dosing Methods

Manual Dosing

  • Measure and add supplements by hand
  • Good for light coral loads
  • Requires discipline and consistency

Dosing Pumps

  • Automated dosing on schedule
  • Ideal for moderate to heavy coral loads
  • Maintains stability
  • Dose small amounts throughout day

Calcium Reactor

  • Dissolves calcium carbonate media with CO2
  • Best for heavy SPS tanks
  • Provides calcium, alkalinity, and trace elements
  • Requires monitoring and adjustment

Kalkwasser (Limewater)

  • Calcium hydroxide solution
  • Dosed as top-off water
  • Raises calcium, alkalinity, and pH
  • Must be dosed slowly

Water Change Best Practices

  • Frequency: 10-20% weekly or 20-30% bi-weekly
  • Match parameters: Temperature and salinity before adding
  • Use quality salt: Reef-specific salt mixes
  • Mix thoroughly: 24 hours before use is ideal
  • Use RODI water: Zero TDS (total dissolved solids)

Common Parameter Problems

Alkalinity Swings

  • Most common issue in reef tanks
  • Causes: inconsistent dosing, coral growth spurts
  • Solution: dose smaller amounts more frequently

Low Magnesium

  • Prevents calcium/alk from stabilizing
  • Often overlooked
  • Solution: test and dose magnesium first

High Nutrients (Nitrate/Phosphate)

  • Causes algae blooms, brown corals
  • Solution: increase water changes, reduce feeding, improve filtration

Ultra-Low Nutrients

  • Can stress corals, cause pale coloration
  • Solution: feed more, reduce nutrient export

Advanced Parameters (Optional)

For advanced reef keepers, these parameters can be monitored:

  • Potassium (K): 380-400 ppm - enhances coral coloration
  • Iodine: 0.06 ppm - supports coral health
  • Strontium: 8-10 ppm - used in coral skeletons
  • ORP (Redox): 300-400 mV - indicates water quality

Note: These are typically maintained through quality salt mix and regular water changes.

Equipment Recommendations

Essential Testing Equipment

  • Refractometer (not hydrometer)
  • Alkalinity test kit
  • Calcium test kit
  • Nitrate test kit
  • Phosphate test kit
  • Digital thermometer

Nice to Have

  • pH meter or monitor
  • Magnesium test kit
  • TDS meter (for RODI water)
  • Hanna checkers (digital testers)

Ready to Build Your Reef?

Now that you understand water parameters, you're ready to maintain a stable, healthy reef environment! Browse our coral collections:

More Resources

Remember: Stability is more important than perfection. Test regularly, make small adjustments, and your corals will thrive!