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Understanding the Role of Magnesium in a Reef Aquarium

MagnesionOften overlooked, Magnesium plays a critical role in the chemical and biological processes in the Marine aquarium. Magnesium is a major element (as opposed to a trace element) and is the third most common element in seawater behind only Sodium and Chloride. Magnesium is an essential element to all organisms for biological functions, and is especially important to organisms that are skeleton building, as Magnesium is a key component of aragonite. Far too often, we find aquarists who are struggling to figure out what is causing issues in their reef aquarium, and Magnesium deficiencies end up being at fault. It is impossible to maintain ideal Calcium and Carbonate levels in salt water without maintaining Magnesium levels as well.  Many folks struggle with keeping their Alkalinity and Calcium to appropriate levels in their marine aquariums, yet never test their Magnesium levels. At levels below natural sea water concentrations of Magnesium (1280-1350 ppm), Calcium and Carbonates will precipitate out with each other in inorganic forms, and dosing either will not achieve the proper results in a low Magnesium environment.

Sources of Magnesium

Quality salt mixes will provide natural seawater concentrations of Magnesium, when properly mixed.  Many Calcium supplements will also provide some level of Magnesium as well, some intentionally, as some products will contain Magnesium as an “impurity” in the source of calcium carbonate used in processing.  Magnesium is also released from the dissolution of media within Calcium reactors.  Magnesium is also available as a stand-alone supplement, in both liquid and powdered forms.  Concentrations of all these sources are highly dependent on the manufacturer’s processes, and the sources of Calcium and Magnesium used.

Dosing Magnesium

Test your water! Before dosing anything, you need to know where you’re starting. Maintaining magnesium levels is a balance between what you are adding, and what is being consumed. Every aquarium is different, and each will consume magnesium at its own individual rate, but you should try to maintain levels between 1250 ppm and 1350 ppm.

For those who are keeping fish only marine aquariums, routine water changes with a quality salt mixture is probably enough to keep Magnesium levels high enough. Chances are you will never have a problem; the element is used up very slowly, and is regularly replenished by the salt mix.

The beginner reef-keeper maintaining live rock and a mix of soft corals, zooanthids, mushrooms and maybe a few stony corals, will typically have a slightly higher magnesium demand than a fish-only tank. Water changes will probably not be enough to maintain ideal levels in these set-ups, but using products that are a combination of minerals usually do the trick and they’re easy to use.  Products like Seachem Reef Complete, Kent Liquid Calcium Reactor, Brightwell Liquid Reef and Red Sea Reef Foundations ABC+ stand out among these products.

Advanced reef aquarists maintaining stony coral dominated aquariums, particularly fast growing SPS corals, will generally have a much higher Magnesium demand. Advanced reef aquarists also tend to be much more hands on with everything that goes into their aquariums, and stand-alone Magnesium supplements tend to be the method of choice. Products like Brightwell’s Magnesion and NeoMag, Seachem’s Reef Advantage Magnesium and Kent’s Tech-M are popular for these aquarists.

Alternative uses for Magnesium

Tech MIn recent years, elevated magnesium levels have been experimented with as a means of controlling nuisance algae. Bryopsis, for example is a very difficult algae to control once established in the aquarium. Many aquarists (myself included) have reported success in the eradication of this pest using Kent Marine’s Tech-M product to raise Magnesium levels upwards of 1800ppm in their aquariums. These levels are maintained for a period of several weeks, until the algae has died off, then allowed to fall back to normal levels. Some say that maintaining elevated levels indefinitely is safe, but I do not feel that this is a good idea. There isn’t much real data on the effects of elevated Magnesium levels for long term use, and there is the potential for toxic affects in the long term. Many aquarists have used this method on other nuisance algaes as well, but there is not definitive guide for what algae magnesium dosing will work on that I am aware of.  This is not a magic bullet, so good, sound husbandry and water quality standards should always be employed.

Hope this clears up some confusion on Magnesium and magnesium dosing, if not please feel free to leave a question or comment.




  1. avatar

    Hi Dave, I live in Bali-Indonesia. I have had salt water aquariums for 8 years now and i have a unique system that none seems to have. I have a semi outdoor 1.75ton set up with my aquarium in a fibreglass pick up truck bath holder pond (from our old Ute car… the pond was never used and sat in our warehouse for about 2 years before my 3m glass aquarium cracked last October). Even when i used to have the 3m tank all was the same except i used 4MH lights and still the stony corals didn’t survive more than a few months. i still use the 1/4 hp chiller on the new system set up and it hovers between 25*c and 27*c I use fresh natural seawater and replenish 60gallons minimum a day (have staff to do this), 1 big skimmer and about 170L sump next to the pond from the old system. The pond is under cover from our verandah but it does get natural sunlight in the morning and afternoon for what i would consider to be about half of the day light hours. I have about 10 large fish mostly tangs and angels as well as clown fish and lots of mandarins, mushrooms and anemones that are hosted by clowns (some are 5years or longer in my system and are huge). I have heaps of cleaner shrimp and other types about 45 in total that eat algae and are all healthy.
    My problem I think is magnesium as it has been creeping up over the past 6 months and today was at 1600. (red sea test kit) it was high a few months ago at 1400 but now its moving up?? All the other parameters are within normal range. All except salinity that is a little high (i use the floating type to test and its just on the white under the green ideal area. I just started to add fresh water today as i had never had an issue with it before and didn’t think that evaporation would be so high. i used to just throw in a few glasses of aqua every few days bit today i put in 5.5l and the salinity moved closer to the ideal area…will add a few more litres tomorrow and see what happens. My clams and stony corals don’t survive for long (1 month) and the red algae on the rocks dies off quickly too after introducing new live rock (2months). The cardinals are breeding, anemones dividing and all are very healthy. I don’t dose with any chemicals because to be honest ca is 500, Kh is 7 and ph 8.2 nitrates ammonia zero.
    I feed the fish with fresh clams, little fish, prawns, blood worms and red sea terta A pellets. I also add baby fish fry powder food a pinch each evening for the filter feeders but all get new salt water directly from the ocean within 1/2 of collecting it and it has all sorts of living things in it. I have never filtered the fresh sea water in 8 years and have never had an issue from it. If possible i would like to do less water changes as its time consuming for my staff and i would prefer to add chemicals if i can figure out how too?
    I wonder if the magnesium is from the ocean water or from the fish food (but i doubt it because i don’t over feed and its a large system) or is it from the fibreglass truck? i tested the sea water because there are many 5star hotels in the vicinity that t my staff take the water (Padma beach) but the mg levels etc. are all ideal?

    Would high mg make my corals and clams die?

  2. avatar

    Felicia, very interesting system, hard to say what is causing the high Mg levels without any dosing or artificial salt mixes being used. Likewise with the problems you are having with clams and stony corals. Light could possibly be an issue, you could also have an issue with phosphates or other contaminants from collecting water from near shore.

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About Dave Acland

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After graduating from Coastal Carolina University with a BS in Marine Science in 1996, I started my professional career in 1997 as an aquarist at Ripley’s Aquarium in Myrtle Beach, SC. This was an amazing experience, in which I gained invaluable hands on training in exhibit design and construction, as well as husbandry skills for a wide range of animals. In 2000 I started working at That Fish Place as one of the staff Marine Biologists, with the responsibility of maintaining one of the largest retail fish holding systems in the world. I presently hold the position of Director of Aquatic Science, where I oversee the operation of our 35,000 gallon retail aquarium systems, and provide technical support for our mail-order and retail store customer service staff. As an aquatic product specialist, I also provide support for our purchasing and marketing departments, as well as contribute web content and analysis. As a Hobbyist I acquired my love of aquariums from my father who was keeping a large aquarium in early 70’s, and set up my first aquarium when I was 12 years old. I have now been keeping aquariums for over 35 years, and through this time have kept more aquariums and types of fish than I can remember. I set up my first Saltwater aquarium in 1992, which led me down the path I still follow today.