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Magnesium is the quiet workhorse of reef chemistry: it keeps calcium and alkalinity in solution so they stay available to your corals instead of precipitating out. Get magnesium right and dosing the other two becomes easy. Pair it with Aragonite A (Calcium) and Aragonite B (Alkalinity) for complete, stable chemistry, and explore the rest of our reef additives and supplements. Magnesium is the foundation. If magnesium is low, calcium and alkalinity will not hold no matter how much you dose. Set magnesium first with Aragonite C, then balance A and B on top of it. Magnesium is consumed slowly, so it usually needs only occasional topping up. Why Magnesium Matters Magnesium sits between calcium and carbonate in the water and stops them from binding together and falling out of solution. When magnesium runs low, you will see calcium and alkalinity drop together no matter how much you add. Aragonite C lets you hold magnesium in the ideal 1300 to 1350 ppm range, which is roughly three times your calcium level. With magnesium locked in, calcium (Aragonite A) and alkalinity (Aragonite B) stay stable and your corals get the steady chemistry they need. Features Concentrated MagnesiumA strong magnesium source that goes a long way per dose. Reef Safe FormulaClean ingredients formulated specifically for marine and reef tanks. Part of a 3-Part SystemWorks hand in hand with Aragonite A (Calcium) and B (Alkalinity). Holds Cal and Alk StableKeeps calcium and alkalinity in solution and available to corals. Made in AustraliaProduced by Quantum to a consistent, lab tested standard. Four SizesFrom a 250 ml nano bottle to a 5 L jug for large or multiple reefs. Choose Your Size Select a size in the tab section below for its full detail sheet. Spec 250 ml 500 ml 1 L 5 L Volume 250 ml 500 ml 1000 ml 5000 ml Best For Nano / pico reefs Small reefs Mid size reefs Large or multiple reefs Value per ml Good Better Great Best SKU AC250 AC500 AC1000 AC5000 Price $22.49 $30.49 $44.99 $139.99 Detailed Size Specifications Click a size below to view its full sheet. 250 ml · Nano 500 ml · Small 1 L · Mid 5 L · Large 250 ml · Nano & Pico Reefs The starter bottle. Ideal for nano and pico reefs, or for trying the Aragonite system before stepping up to a larger jug. Specification Value Volume 250 ml SKU AC250 Role Magnesium (Part C) Best For Nano and pico reef tanks Price $22.49 500 ml · Small Reefs A practical everyday size for small reef systems keeping magnesium topped up. Specification Value Volume 500 ml SKU AC500 Role Magnesium (Part C) Best For Small reef tanks Price $30.49 1 L · Mid Size Reefs The best balance of value and shelf life for most established reef tanks. Specification Value Volume 1000 ml (1 L) SKU AC1000 Role Magnesium (Part C) Best For Mid size reef tanks Price $44.99 5 L · Large or Multiple Reefs The bulk jug for large, heavily stocked SPS systems or hobbyists running several reef tanks. The lowest cost per ml. Specification Value Volume 5000 ml (5 L) SKU AC5000 Role Magnesium (Part C) Best For Large or multiple reef tanks Price $139.99 Which Size Is Right for You? 250 ml — Nano and pico reefs, or a low commitment way to start the Aragonite system. 500 ml — Small reefs keeping magnesium in range. 1 L — The sweet spot for most mid size reefs. Best mix of value and freshness. 5 L — Large SPS systems or multiple tanks where you want the lowest cost per ml. How to Dose Always test before you dose. Use a quality magnesium test kit, then add Aragonite C to bring magnesium into the 1300 to 1350 ppm range. Magnesium is used up slowly, so once you reach your target you usually only need to top it up occasionally rather than daily. Follow the dosing guide on the bottle and adjust to your own test results. Keep calcium, alkalinity, and magnesium in balance by dosing Aragonite A, B, and C together, and pick up a reliable kit from our water testing collection. FAQ Why can I not keep my calcium and alkalinity up?Low magnesium is the usual culprit. When magnesium is below range, calcium and carbonate bind and precipitate, so they fall no matter how much you dose. Set magnesium with Aragonite C first. What magnesium level should I aim for?Most reef keepers hold 1300 to 1350 ppm, roughly three times the calcium level. Test, then dose Aragonite C to reach that range. How often do I dose magnesium?Less often than calcium or alkalinity. Magnesium is consumed slowly, so after you set it you usually just top up occasionally based on testing. Do I need all three Aragonite products?Yes. Calcium, alkalinity, and magnesium work as a set. Magnesium is the foundation that lets calcium and alkalinity stay where you want them.
