Foundation C Magnesium 500ml — Keeps Alkalinity and Calcium in Balance - Red Sea

Foundation C Magnesium from Red Sea is a blend of magnesium salts. It is the least glamorous of the three foundation elements, and the one that explains the most problems when it runs short. 500ml liquid bottle.

Red Sea explains the Foundation program in its own words:

And for a longer walk-through of the whole Reef Care Program — major elements, trace elements, algae management and coral nutrition — this independent explainer covers the ground well:

The element that holds the other two up

Magnesium goes directly into coral skeleton as magnesite (MgCO₃). But its most important job is elsewhere: when magnesium is low, alkalinity drops — Red Sea lists it explicitly among the causes of unstable alkalinity. A reef keeper dosing buffer week after week with nothing holding usually has a magnesium problem, not an alkalinity problem.

It also affects the quality of the skeleton itself: under low magnesium or low strontium conditions, the skeleton forms with more calcite — a more brittle structure, more susceptible to damage.

Specifications

• Composition: blend of magnesium salts
• Concentration: 1ml raises magnesium in 100 litres (25 gal) by 1ppm
• Daily maximum: 10ppm of magnesium per day
• Sizes: 250ml, 500ml (this listing), 1L, 5L and 1kg powder
• Suitable for: reef aquariums with SPS, LPS and soft corals · dosing pump compatible

Worth knowing: magnesium is dosed in far larger volumes than calcium, since it takes 1ml per 1ppm against 1ml per 2ppm for calcium, and targets sit around 1300 mg/l. A magnesium bottle empties faster than most people expect.

Target levels for your reef tank

Before testing anything, check salinity: Red Sea notes that a 1 ppt rise from evaporation raises magnesium by roughly 40ppm and calcium by about 13ppm. Adjust, wait 10 minutes, then test.

How to dose: the manufacturer's method

The "1ml = 1ppm" figure converts a measured gap into millilitres; it is not a daily dose. Red Sea'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, ahead of alkalinity and calcium, with 10 minutes between each.
• Where to add it: into the sump, or slowly into a high-flow area.

Goes well with

Magnesium is the insurance policy on the other two. It lets Foundation A Calcium+ and Foundation B KH/Alkalinity coexist in the water without precipitating against each other. Dosed together in the right order, the three hold; dosed in isolation, they work against each other.

To start with all three, the Foundation ABC 3x250ml trio. Once the base is stable, Trace Colors ABCD brings the 31 trace elements tied to coral colour.

Frequently asked questions

Why will my alkalinity not hold despite dosing?

Low magnesium is one of the causes Red Sea lists. Magnesium keeps calcium and carbonates from precipitating; without it, the buffer you add disappears instead of raising alkalinity. Test magnesium before increasing your Foundation B dose.

What magnesium level should I target?

Between 1100 and 1390 mg/l depending on tank type — 1350 for a mixed reef, 1280 for SPS dominant, 1220 for ULNS. See the table above.

How much can I raise magnesium per day?

By 10ppm at most, per Red Sea. A 200ppm gap therefore takes around three weeks to close — slow, and deliberately so.

Should I check salinity before testing?

Yes. A 1 ppt rise raises magnesium by roughly 40ppm. Adjust salinity, wait 10 minutes, then test.

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.