Foundation C Magnesium 1kg — Magnesium Powder for Reef Tanks - Red Sea

Foundation C Magnesium powder from Red Sea is the blend of magnesium salts in dry form. 1kg bag. Since magnesium is the element of the trio dosed in the largest volumes, it is often the first one where powder pays off.

Powder and liquid: different strengths

• Dry powder: 1g raises magnesium in 100 litres (25 gal) by 1.34ppm
• Ready-to-use liquid: 1ml raises magnesium in 100 litres by 1ppm

Never carry a calculation from one format to the other.

Making your stock solution

Red Sea's recipe: 600g of powder per litre of RO water. A 1kg bag yields 1.7 litres of stock solution, dosed afterwards like the shelf liquid: 1ml per 100 litres raises magnesium by 1ppm. It is also the only way to run the powder through a dosing pump.

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
• Powder concentration: 1g raises magnesium in 100 litres (25 gal) by 1.34ppm
• Stock solution: 600g per litre · 1kg yields 1.7L
• Daily maximum: 10ppm of magnesium per day
• Sizes: 250ml, 500ml, 1L, 5L liquid · 1kg powder (this listing)
• Suitable for: SPS, LPS and soft coral reef tanks

The powder concentration (1.34ppm per gram) comes from CoralVue, Red Sea's official North American distributor. Red Sea's own Foundation manual publishes the 600g/litre stock solution recipe. If you see a different value elsewhere, check the back label on your bag — that is the authority — and write to us, we will update the page.

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.

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.
• 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+ 1kg and Foundation B KH/Alkalinity 1kg coexist without precipitating against each other.

If mixing solutions is not your thing, Foundation C Magnesium 1L liquid doses as-is.

Frequently asked questions

How much solution does a 1kg bag make?

1.7 litres of stock solution, at 600g per litre of RO water. That solution then doses like the Red Sea liquid: 1ml per 100 litres raises magnesium by 1ppm.

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.

Can I add the powder straight to the tank?

Red Sea publishes a stock solution recipe precisely so you do not have to, and it is the only way to use a dosing pump.

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.