Foundation C Magnesium 1L — 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. 1 litre liquid bottle.

The element that holds the other two up

Magnesium goes 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 one.

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
• Daily maximum: 10ppm of magnesium per day
• Sizes: 250ml, 500ml, 1L (this listing), 5L and 1kg powder
• Suitable for: SPS, LPS and soft coral reef tanks · dosing pump compatible

Magnesium is dosed in far larger volumes than calcium: 1ml per 1ppm against 1ml per 2ppm for calcium, with targets around 1300 mg/l. It is usually the first bottle of the trio to empty, which is why the litre makes sense here even on a mid-sized tank.

Target levels for your reef tank

Before testing, check salinity: 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

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, 10 minutes between each.
• Where to add it: into the sump, or slowly into a high-flow area.

Which size to choose

Since magnesium goes fast, the litre is often the right starting point even on a mid-sized tank. For a nano, the 500ml. For a large system or a dosing pump running continuously, the 5L.

Goes well with

Magnesium is the insurance policy on the other two: it lets Foundation A Calcium+ 1L and Foundation B KH/Alkalinity 1L coexist without precipitating against each other.

Frequently asked questions

Why will my alkalinity not hold despite dosing?

Low magnesium is one of the causes Red Sea lists. Without it, the buffer you add precipitates 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.

How much can I raise magnesium per day?

By 10ppm at most. 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, 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.