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Magnesium Calculator

Enter your aquarium details to calculate the amount of magnesium supplement needed.

Gallons Liters

Do not dose all at once. Doing so can cause extreme changes in water chemistry. Use smaller amounts for 1-2 days (or more) to make gradual changes. Monitor pH closely and do not allow for swings more than +/- 0.20. Test your parameters daily until levels are back in line.

Magnesium Dosing Calculator

Magnesium plays a critical role in reef aquariums by helping maintain proper calcium and alkalinity balance, supporting coral health, and stabilizing overall water chemistry. This calculator helps you determine the right dose for your system using your preferred magnesium supplement.

Start Slow: Recommended Usage

Note: The values generated by this calculator are suggested starting points and should be adjusted based on your tank’s needs.

We recommend beginning with half the calculated dose and testing your magnesium level 24 hours later. From there, adjust slowly as needed to reach your target.

Avoid dosing the full amount all at once. Rapid changes in magnesium can destabilize water chemistry and stress corals. Instead, spread the dose over 1–2 days or more for a safer, gradual correction.

Monitor pH closely during this process and avoid swings greater than ±0.20. Test your parameters daily until levels are stable.

How This Calculator Works

By entering your current magnesium level, target level, and tank volume, the calculator determines the precise dose needed based on the specific product you’re using.

Supported Magnesium Supplements

This calculator currently supports:

  • Red Sea Reef Foundation C (Mag)
    A balanced magnesium supplement designed for use with the Red Sea Reef Care Program.
  • Brightwell Magnesion
    A highly concentrated liquid magnesium additive for precise dosing and reef supplementation.
  • Fauna Marin ELEMENTALS MAG
    Pure magnesium concentrate tailored for the Balling Light system or standalone use.
  • Fritz RPM Liquid Magnesium
    Liquid magnesium designed to replenish and maintain ideal magnesium levels in reef aquariums.
  • Oceans Blend Magnesium
    A straightforward magnesium solution ideal for reef tanks with moderate to high demands.
  • E.S.V. Magnesium Chloride
    A dry magnesium chloride additive used to correct magnesium levels and maintain balance in two-part or Balling systems.

When to Dose Magnesium

Aim for a magnesium range of 1250–1500 ppm in most reef systems. Dose when levels fall below your target, especially in coral-dense tanks where magnesium is consumed alongside calcium and alkalinity. Always test before and after dosing to confirm changes.

Tips for Best Results

  • Test magnesium before and 24 hours after dosing
  • Track alkalinity and calcium alongside magnesium for balanced chemistry
  • Dose incrementally and avoid rapid corrections
  • Keep dosing records to fine-tune ongoing maintenance