Brief Look Into Magnesium
I ignored magnesium for a long time. Calcium and alkalinity got all my attention because those are the two numbers everyone tells you to watch, and magnesium just sat there in the background. Turns out it was doing more work than I gave it credit for.
Here’s what it actually does: magnesium keeps calcium and carbonate from bonding together and dropping out of your water as chalky white gunk. Without enough of it, your calcium and alkalinity won’t stay where you put them no matter how much you dose. I learned this the hard way when my magnesium dipped below 1200 and I spent a week chasing calcium numbers that refused to hold. I’d dose, test, see the number drop, dose again. It felt like pouring water into a bucket with a hole in the bottom, which is basically what was happening at a chemical level.
My tank sits around 1300 ppm, which is close to natural seawater. That 1250 to 1350 range is the sweet spot most people aim for, and in my experience it’s where things just work. Drop below that and you start seeing the signs: white buildup on heaters and powerheads, calcium and alkalinity that won’t stabilize, and eventually corals that stop growing. Below 1000 things get ugly fast. Growth stalls, colors fade, and LPS start receding from their bases.
It’s not just keeping the peace between calcium and alkalinity, though. Corals actually fold it into their skeletons as they grow, so they’re consuming it alongside calcium, just slower. Coralline algae, clams, urchins, and gorgonians all use it too, so in a tank with a lot of life it adds up over time. It also helps zooxanthellae do their thing, which is where your corals get most of their color and energy. When magnesium is low, don’t be surprised if everything starts looking a little washed out.
Too much magnesium is a problem too, though it’s harder to accidentally overdose. Anything above 1500 ppm and you’re asking for trouble. It throws off your calcium chemistry, and I’ve seen reports of soft corals dissolving and SPS losing tissue at those levels. You’ll also hear people say jacking magnesium up past 1800 kills nuisance algae. Maybe it does, but it’ll probably kill your corals first. Not worth it.
Magnesium doesn’t get consumed as fast as calcium or alkalinity, so it’s easier to keep in line. For a lot of reefers, regular water changes with a decent salt mix are enough. When that’s not cutting it, magnesium chloride is the usual supplement, sometimes mixed with magnesium sulfate to keep the ionic balance right. I use a three-part dosing system that handles magnesium along with calcium and alkalinity, which keeps me from having to think about it much. I test once a week and rarely need to adjust.
Like everything else in reef chemistry, go slow. I wouldn’t move more than 100 ppm in a day. Your tank didn’t get here overnight and it won’t get fixed overnight either.
Sources
- Bulk Reef Supply - “Magnesium: The Magic That Holds Your Reef Tank Together”: bulkreefsupply.com
- Charterhouse Aquatics - “Understanding Magnesium (Mg) in a Reef Tank” (2024): charterhouse-aquatics.com
- Tidal Gardens - “Magnesium”: tidalgardens.com
- ATI North America - “All You Need to Know About: Magnesium” (2021): atinorthamerica.com
- Orphek - “Magnesium (Mg) and Its Role in the Reef Aquarium”: orphek.com
- Fauna Marin - “Magnesium” knowledge base: faunamarin.de
- Reef Builders - “The Importance of Calcium, Alkalinity, and Magnesium in a Reef Tank” by Michael Paletta (2023): reefbuilders.com
- Manta Systems - “Learning About the Importance of Magnesium”: mantasystems.net