Magnesium is actually a very important part of sea water, being the third most abundant element behind chlorine and sodium. Until recently not much has been done or thought about its level within a reef aquarium, and in some cases with synthetic salt water mixes it has been present in relatively low concentrations.

Before you can even think of playing around with the magnesium concentrations, a test kit that will enable magnesium concentrations of the water to be measured is required. It is bit of a fruitless task to be adding something when the actual concentration is not known to start with. The kits are relatively easy to use, using the titration/colour change technique to determine the concentration that calcium test kits use. Once a test kit has been obtained, then there are basically four techniques that can be used to add magnesium.

• Water changes
• Dolomite in a calcium reactor
• Commercial additive
• DIY additive

Water Changes

Water changes are most likely already being used in the reef aquarium system, but for other reasons. This is not the best technique, as the water used to make the change has to have a suitable level within it. And if it is not elevated above the concentration wanted to be reached, then it will not a difference. If synthetic salt mixes are used, it can be even harder as some of the mixes can be lower in the magnesium department. There are better ways to do it.

Dolomite in Calcium Reactor

If a calcium reactor is already used on the reef system, then it can simply be a matter of adding a small amount of dolomite to the substrate used within the reactor. Dolomite is a mineral containing both calcium and magnesium carbonate, CaMg(CO₃)₂. As it is dissolved in the lower pH environment in the reactor, both the magnesium and calcium are released into the water. The amount used in the reactor will have to be adjuested so that the correct balance can be found between usage in the aquarium system and that supplied by the reactor. A good percentage to start with is around 20% of the substrate contained within the reactor, then just increase or decrease the percentage as required by monitoring the magnesium concentration.

Commercial Additives

There are quite a few commercial additives out there now that can be used. Just follow the instructions on the label, which will give a dosing rate for a given volume to raise the level by a certain amount. These are fine, but there is a way to do this that is exactly the same, but a fraction of the price. You also get to flex those DIY muscles a little too, so it it time to warm them up ;-).

D.I.Y. Additive

Here is a nice little magnesium additive secret, one that I am sure many reef aquarium additive companies don't want you to hear about, Epsom Salt. Yep, the stuff that your grandmother uses for tired muscles, aches, pains and a mild laxative has a use in your reef tank. Epsom Salt, or MgSO₄.7H₂O, is easily available from the local chemist, pharmacy or drug store (depending on what you call it) and in some cases a supermarket. A half kilogram box should not cost more than a couple of dollars, which sure beats twenty or more for a commercial additive.

The dosing rate is simply: to increase the magnesium concentration by 100 ppm, add 1 gram of Epsom Salt per litre of water. As with anything change with the water parameters, take it slowly, and any sensitive corals will soon show effects when things are altered too fast for them. The following is the system that can be use to add magnesium using Epsom Salt.

1. Calculate the total system volume, including tank, sump, and other water containing equipment. Don't forget that rock and sand takes up water volume and that tanks are not filled right to the top.
2. Measure the magnesium level currently of the water, determine what level is being aimed for, and calculate the concentration increase that is required to reach this level. Note that natural sea water has a level of around 1,300 ppm. If the increase required is more than 200 ppm then consider doing the increase in several steps over several days.
3. To obtain the mass of Epsom Salt to add in grams, multiply the system water volume (in litres) by the dosing rate (1 gram per litre), then by the concentration increase (ppm) and divide by 100 (ppm). As an equation this looks like this:
   Mass Epsom Salt (grams) = 1 (grams/litre) * Water Volume (litre) * Magnesium Concentration Increase (ppm) / 100 (ppm)
4. Weight or measure out the mass of Epsom Salt required. Because this is on a mass basis, either easy access to an accurate weighing scale or an one off access is required. If one is available, then just weigh it out as required. If not, then this be solved by using a small measuring cup, spoon or plastic container and determine how much one measure of Epsom Salt from this weighs. Be careful if the type of Epsom Salt used is changed, as it's density can change and therefore the correct mass will not be measuring out in the future.
5. Take out about a litre of water from the reef tank into a plastic container and add the required mass of Epsom Salt to the water. Stir until it is fully dissolved. Increase or decrease the dissolving water volume as required for the amount to be added.
6. Pour magnesium water into the tank in an area of high water flow and turbulence. This will allow it to be mixed in rapidly with the water. In front of a pump outlet or inlet is a good place for this. Avoid pouring into areas where it will come into contact with corals, invertebrate etc before mixing in correctly.
7. If a over 200 ppm concentration increase is required, then wait until the next day before adding the next amount.
8. Test the water several hours later, if not the next day, to check the magnesium level. Note that it is not a good idea to check it straight away, as it takes time for it to mix in and equilibrate.

But will using this type of additive could cause any long-term effects or problems? Note that the magnesium exists as a sulphate salt in Epsom Salt, in a one to one ratio. If Epsom Salt is added to the water to increase the magnesium levels then there will also be an increase in the sulphate levels. To determine if this will be much of a problem, the various species concentrations in natural water have to be considered:
Magnesium, Mg = 1,300 ppm
Sulphur, S = 900 ppm
Chlorine, Cl = 19,500 ppm

By a rough approximation, it can be assume that all the sulphur present is found as sulphate, SO₄^2-. Note that both the magnesium and sulphate exist in comparative concentrations, around 1,000 ppm. Therefore for a given increase in the magnesium concentration achieved by using Epsom Salt, a comparative increase in the sulphate concentration will also occur. In contrast if the magnesium additive magnesium chloride (MgCl₂) is used the effect will be smaller. This results from the factor of fifteen higher concentration of chloride over that of magnesium.

Now for as how this applies to our reef aquariums, I think that the increase in sulphate concentrations should not cause any problems. Low magnesium levels cause more, and the amount of Epsom Salt required to maintain the magnesium concentrations once they are up to the correct level is small. But having said that, it is better to strive for the conditions that our system inhabitants are adapted to and have evolved in. If a source of magnesium chloride that is not expensive and can be afforded can be found then go ahead and use it. Although because of the ease of access, price and simplicity of use, Epsom Salt is a great place to start, or even end with, maintaining magnesium concentrations.

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