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Brightwell Aquatics - Magnesion-P (400g)

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Magnesium Supplement for All Marine Fish & Reef Aquaria


  •  High-purity, anhydrous powdered magnesium blend.
  •  Provides magnesium, which helps regulate the balance between calcium and carbonates in aquarium water, and is a component of aragonite, the mineral secreted by reef-building organisms such as corals, clams, and their allies to form skeletal material.
  • Contains about 300% the magnesium of almost all other products on the market because it is anhydrous (without water).
  • Contains no unwanted organic material such as EDTA, gluconate, or polygluconate
  • Formulated by a marine scientist.

Technical Background

Magnesium plays a vital role in reef aquaria: it helps maintain calcium and carbonate (composing the majority of measurable alkalinity) concentrations by limiting unwanted inorganic precipitation with each other. In the absence of adequate magnesium ions, calcium and carbonate ions bond rapidly, depleting their concentrations in the system and creating the false impression that heavier supplementation is required to re-establish the desired concentrations. No matter how much calcium is added, its natural seawater concentration of 412 ppm will never be sustained until the magnesium concentration begins to approach 1,290 ppm. Once the balance between calcium and magnesium has been established, subsequent calcium supplementation yields immediate positive results. Keeping magnesium within a range of 1,290-1,320 ppm will maintain the desired balance, and provides enough of the element to reef-building organisms for the formation of aragonite (~0.10% Mg by weight).

Instructions and Guidelines

Basic: Dissolve 2.5 grams (~3/4 teaspoon) of product per 20 US-gallons of aquarium water in 16-fl. oz. of fresh water; add every other day or as needed to maintain the magnesium concentration within a range of 1,290 - 1,320 ppm. When used in this fashion, 1,200 g treats up to 9,600 US-gallons (36,340 L).

Advanced: First raise the magnesium level using the basic instructions above.  Once desired level has been acquired, measure aquarium's magnesium concentration at the same time each day over a one- to two-week period to determine the daily rate of magnesium uptake (i.e. the decrease in magnesium daily).  Determine the amount of Magnesion-P to add using this formula:  Daily Amt (grams) = Daily Loss (in ppm) x Tank Volume (gallons) / 64.  Alternately to measure in liters, use this formula:  Determine the amount of Magnesion-P to add using this formula:  Daily Amt (grams) = Daily Loss (in ppm) x Tank Volume (liters) / 243.  


For example:  If you determine that you need to raise the magnesium level in your 50 gallon aquarium by 10 mg/L or 10 ppm, you would add 10 x 50 / 64 or 7.8 grams or about 2.5 teaspoons of powder in as much water as it takes to fully dissolve (about 16 oz. per 2.5 grams or about 48 oz.) of water and mix until clear and cool.  Then add that amount to your aquarium.


Caution: Keep out of reach of children. Not for human consumption.  Wear rubber gloves and goggles when handling.  Contents may get very hot when water is added!



Magnesium sulfate (anhydrous), Magnesium chloride (anhydrous)


Guaranteed Analysis

Magnesium (min) 24% (240,000 ppm)

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