Seachem Reef Advantage Magnesium – Concentrated Magnesium Supplement for Reef Aquariums

Restore Magnesium Without Disrupting Your Reef

Kalkwasser use, carbonate overdosing, and low-magnesium salt mixes quietly drain magnesium — and once it drops, calcium and alkalinity become nearly impossible to hold.

Reef Advantage Magnesium is a powerful, concentrated (80,000 ppm) blend of magnesium, chloride, and sulfate salts that raises depleted magnesium without disrupting the ionic ratios of natural seawater. It's safe for delicate sea creatures and, unlike other products on the market, contains no harmful ammonia.

That careful balance of salts reflects its origin: Seachem Laboratories, a science-driven chemistry company founded in 1979 in Madison, Georgia, where products are developed in-house by chemists. Reef Advantage Magnesium is engineered to raise magnesium while preserving natural seawater ionic ratios and avoiding ammonia, and it is designed to work as the magnesium component of Seachem's coordinated reef system — the element that lets the line's calcium and alkalinity supplements actually stay in range.

1. Beginners: dissolve 5 g (1 level teaspoon) per 80 L (20 gallons) in at least 250 mL (1 cup) of freshwater, twice a week.
2. Advanced keepers: dose toward 1,200-1,350 mg/L, where each 5 g/80 L raises magnesium by about 5 mg/L.
3. Add magnesium and carbonate supplements on separate days or at least 30 minutes apart.

Lock in stable calcium and alkalinity. Add to cart.

Why Seachem Reef Advantage Magnesium

• The element that stabilizes the reef: restores depleted magnesium, the parameter that makes it possible to hold calcium and alkalinity in range.
• Concentrated and efficient: a powerful, concentrated 80,000 ppm blend of magnesium, chloride, and sulfate salts, so small doses go a long way.
• Preserves natural seawater balance: raises magnesium without drastically impacting natural ionic ratios, and contains no harmful ammonia — safe for delicate sea creatures.
• Part of Seachem's reef system: designed to be dosed with the line's calcium and carbonate supplements (on separate days or 30 minutes apart) for balanced water chemistry.

Best For

Reef aquariums where calcium and alkalinity are hard to hold steady — especially tanks using kalkwasser, dosing carbonate additives, or running low-magnesium salt mixes — housing SPS, LPS, and other calcifying corals and invertebrates.

FAQ

How do I know how much to dose? Test your magnesium and dose toward 1,200-1,350 mg/L. Beginners can use 5 g (1 level teaspoon) per 80 L (20 gallons) twice a week, dissolved in at least 250 mL of freshwater; each 5 g/80 L raises magnesium by about 5 mg/L. Do not exceed 25 g/80 L per day.

Can I mix it with my carbonate supplement? No. Do not directly mix Reef Advantage Magnesium with any carbonate supplement. Dose magnesium and carbonate additives on separate days or at least 30 minutes apart.

Is it reef-safe? Yes. It is safe for delicate sea creatures and, unlike some products, contains no harmful ammonia, making it suited to coral and invertebrate reef systems.

What It Does

A powerful, concentrated (80,000 ppm) blend of magnesium, chloride, and sulfate salts that helps increase depleted magnesium levels without drastically impacting the ionic ratios found in natural seawater. It is safe for delicate sea creatures and does not contain harmful ammonia. Magnesium deficiencies are generally caused by using kalkwasser, overdosing carbonate additives, or having low levels of magnesium in the salt mix itself.

Directions

Beginner: Use 5 g (1 level teaspoon) per 80 L (20 gallons) of aquarium water twice a week. Dissolve this mixture in at least 250 mL (1 cup) of freshwater. Advanced: Carefully check the magnesium levels and then follow addition instructions in “Beginner” until the magnesium has been adjusted to 1,200 - 1,350 mg/L. Each 5 g/80 L will raise the magnesium levels by approximately 5 mg/L. The overall amount or dose frequency can be easily adjusted to suit individual needs, but be careful not to exceed 25 g/80 L per day. Use as needed to maintain optimal magnesium levels. Expert: After identifying the depletion rate of magnesium within the aquarium, set up a continuous drip system. Use the following formula to determine how much to add to your top-off water: g=vm/80 (g=grams to add to top off water, v=volume of tank in liters, m=amount to raise magnesium in mg/L). For example, if you want to raise magnesium by 10 mg/L in a 200 liter tank, then you would add (200x10)/80=25 g into the top-off water. Do Not Overdose: Do not directly mix this solution with any carbonate supplement, and it is highly recommended that magnesium and carbonate additives are added on separate days or at least 30 minutes apart. Slight clumping of this product may occur but it does not impact its effectiveness; any clumps that remain visible will break up with ease.

Available Sizes

2.2 lb