Foundation C Magnesium 5L — Bulk Size for Large Reef Aquariums - Red Sea

Foundation C Magnesium 5L from Red Sea is the blend of magnesium salts in a 5 litre jug. Since magnesium is the element of the trio dosed in the largest volumes, it is usually the first one people move up to bulk size for.

Why magnesium goes so fast

It is arithmetic: it takes 1ml per 1ppm of magnesium against 1ml per 2ppm of calcium — and magnesium targets sit around 1300 mg/l against 450 for calcium. When one bottle of the trio empties before the others, it is almost always this one.

The element that holds the other two up

Magnesium goes into the skeleton as magnesite (MgCO₃), but its main job is elsewhere: when magnesium is low, alkalinity drops — Red Sea lists it among the causes of unstable alkalinity. A tank where the buffer never holds often has a magnesium problem. It also affects skeleton quality: under low magnesium or strontium, the skeleton forms with more calcite, which is more brittle.

Specifications

• Composition: blend of magnesium salts
• Concentration: 1ml raises magnesium in 100 litres (25 gal) by 1ppm — identical to the other liquid sizes
• Daily maximum: 10ppm of magnesium per day
• Sizes: 250ml, 500ml, 1L, 5L (this listing) and 1kg powder
• Suitable for: SPS, LPS and soft coral reef tanks · dosing pump compatible

Target levels for your reef tank

Before testing, check salinity: a 1 ppt rise from evaporation raises magnesium by roughly 40ppm. Adjust, wait 10 minutes, then test.

How to dose: the manufacturer's method

1. Bring Ca, Alk and Mg to their targets.
2. Run 4 days without dosing anything, topping up evaporation with fresh water.
3. Test, work out the dose to return to target, add it.
4. Divide by 4: that is your daily dose for the week.
5. Retest weekly and adjust.

Three rules not to skip

• Daily maximums: magnesium 10ppm, alkalinity 0.5 meq/l (1.4 dKH), calcium 20ppm.
• Dosing order: magnesium goes first, 10 minutes ahead of alkalinity, then calcium. With three dosing pumps, stagger the schedules.
• Where to add it: into the sump, or slowly into a high-flow area.

Need a smaller size?

For a mid-sized tank, the 1L. For a nano or to start out, the 500ml. Same liquid, same concentration — only the volume changes.

Goes well with

Magnesium is the insurance policy on the other two: it lets Foundation A Calcium+ 5L and Foundation B KH/Alkalinity 5L coexist without precipitating against each other.

Frequently asked questions

Why does magnesium empty faster than calcium?

Because it takes twice as much per ppm (1ml per 1ppm against 1ml per 2ppm for calcium) and the target is three times higher. It is normal, and it is why bulk size usually makes sense for magnesium first.

Why will my alkalinity not hold despite dosing?

Low magnesium is one of the causes Red Sea lists: without it, the buffer precipitates instead of raising alkalinity. Test magnesium before increasing your Foundation B dose.

How much can I raise magnesium per day?

By 10ppm at most. A 200ppm gap therefore takes around three weeks to close.

Why buy from ReefSolution

Aquarium ReefSolution is an authorized Red Sea dealer. Our team works in marine aquariums in Laval and knows the Reef Care Program from real tanks. We ship across Canada and offer in-store pickup.