Measuring magnesium in reef tanks - what for?

In direct form, magnesium plays a rather subordinate role in your aquarium, because it is consumed by only a few organisms. These include, for example, calcareous algae, soft corals, gorgonians, sea urchins, starfish and mangroves. Stony corals do not require magnesium ions for the formation of their skeleton.

Nevertheless, magnesium ions are important components of various biochemical reactions and processes, such as photosynthesis. In addition, magnesium indirectly ensures a more stable calcium supply by preventing the precipitation of calcium carbonate and thus stabilizing the calcium and KH values. Therefore, you should make sure that the magnesium concentration of your aquarium water is in the range of 1250 to 1350 mg/l.

Adjusting magnesium
Depending on your stock, a weekly water change with a high quality sea salt of 10% may be sufficient to ensure a sufficient supply of magnesium.

If magnesium levels fall below the optimal range (< 1,200 mg/l), you should slowly increase them. To do this, you can either use commercially available ready-to-use products, such as Tropic Marin Bio-Magnesium, or prepare your own stock solution.

To prepare the stock solution, dissolve a total of 836 g of magnesium chloride hexahydrate (MgCl₂*6H₂O) in one liter of osmosis water. The addition of 10 ml of the prepared solution will increase the magnesium concentration in 100 liters of your aquarium water by 10 mg/l.

If you want to increase the magnesium concentration in your 300 liter aquarium from 1,200 to 1,320 mg/l, for example, you have to add 360 ml of the stock solution. However, you should also spread the dosage over a few days so that the increase in magnesium concentration does not occur too abruptly.

If, on the other hand, the magnesium concentration in your tank is too high, temporarily stop adding magnesium or carry out partial water changes (for very high values > 1600 mg/l).