- Sku: AC250
- Vendor: Quantum USA
Quantum Aragonite C - Magnesium
Quantum Aragonite C is a concentrated liquid magnesium supplement and the third part of Quantum's three part Aragonite reef dosing system. 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.
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
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 & 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.
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Quantum Aragonite C - Magnesium
