Seachem Reef Advantage Magnesiumis a concentrated blend of magnesium, chloride, and sulfate salts that restores depleted magnesium while keeping ionic ratios close to natural seawater. Magnesium is the element that lets you hold calcium and alkalinity at the same time, so when it runs low your reef numbers get hard to control and pH can sag. Unlike many liquid magnesium supplements, this one contains no ammonia. Browse our full range of reef additives and supplements and the rest of the Seachem lineup.
Why Reefers Use It
- Keeps calcium and alkalinity in balance.Proper magnesium is what lets both stay where you want them.
- No ammonia.A common contaminant in liquid magnesium supplements, left out here.
- Natural ratios.Restores magnesium with minimal impact to the other ions in your saltwater.
- Fixes the usual culprits.Counters magnesium loss from kalkwasser, heavy carbonate dosing, or a low-magnesium salt mix.
How to Use
Dissolve 5 g (1 level teaspoon) in at least 250 mL (1 cup) of fresh water, then add it to the tank for every 80 litres (about 20 US gallons), twice a week. Each dose raises magnesium by roughly 5 mg/L. Do not exceed 25 g per 80 L per day, since too much magnesium can pull down carbonate alkalinity. Test magnesium regularly and adjust. Do not mix Reef Advantage Magnesium directly with a carbonate or alkalinity supplement; add them on alternate days or at least 30 minutes apart.
Choose Your Size
| Size | Best for |
|---|---|
| 300 g | nano and single reef tanks |
| 600 g | a single reef tank, regular dosing |
| 1.2 kg | larger reefs and fish rooms |
Specifications
| Brand | Seachem |
| Type | Reef magnesium supplement |
| Composition | Concentrated magnesium, chloride, and sulfate salts |
| Ammonia-free | Yes, unlike many liquid magnesium supplements |
| Raises | About 5 mg/L magnesium per 5 g dose in 80 L |
| Form | Powder (measuring scoop included) |
| Water type | Reef and saltwater only |
| Dose | 5 g (1 tsp) per 80 L (about 20 gal) twice weekly, dissolved first |
| Sizes | 300 g, 600 g, 1.2 kg |
Recommended Pairings
- Seachem Reef Advantage Calcium to maintain calcium alongside your magnesium.
- Seachem Reef Buffer to hold alkalinity, the third leg of reef chemistry.
- Magnesium and alkalinity test kits so you can dose to your numbers.
- More reef additives and supplements as your system matures.
FAQ
Why does magnesium matter in a reef tank?
Magnesium keeps calcium and carbonate alkalinity from precipitating out of solution. Without enough of it, you cannot hold both calcium and alkalinity, and pH can drop.
What magnesium level should I target?
Aim for the natural seawater range, roughly 1250 to 1350 ppm, and test regularly to dose to that target.
Can I mix it with my alkalinity supplement?
No. Dose magnesium and carbonate supplements on alternate days or at least 30 minutes apart so they do not react.
How is this better than a liquid magnesium supplement?
It contains no ammonia, which is a common contaminant in liquid magnesium products, and it is concentrated for efficient dosing.
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