Magnesium: Magnesium is HIGH. Just wait and let it drop by itself to about 1300 ppm. For more information consult this article.

Calcium and Alkalinity: Zone 2

Zone 2 is also an easy problem to correct, and is very common.   In this case, both calcium and alkalinity are on the low side of normal.   If you take any tank in the red target zone, and let corals and coralline algae grow (i.e., calcify), then you will move into zone 2.   Just as in the case for Zone 1, you will move downward in a fashion such that you parallel the lines bordering the zone.   That fact is why it is so common to have this problem: anyone not adding enough calcium and alkalinity to balance demand in the tank will likely enter this zone.

To correct this situation back to the red target zone, one wants to add calcium and alkalinity in a balanced fashion.   Fortunately, there are many ways to achieve this balance, such as with limewater (kalkwasser), calcium carbonate/carbon dioxide reactors, and the many balanced two-part calcium and alkalinity supplements (such as B-ionic, C-Balance, etc.).   The more that you add of these, the further upward your correction will go (the blue arrow in the figure below).   Of course, adding too much will push you into zone 1, but if that happens you can just sit back and watch it drop back to the target zone.

Of course, you can add calcium and alkalinity that are not “balanced” (that is, not tied to one another in any specific ratio). For example, adding calcium chloride and sodium bicarbonate will work fine, but you must fine-tune exactly how much of each you add. Consequently, more careful monitoring of calcium and alkalinity levels is necessary if you go this route.

Remember that manufacturer recommendations are based on maintaining a tank, not in making substantial corrections. To make such corrections, you may need to add much more than is recommended. If you are adding a balanced additive (and the pH is not getting out of the range of about 7.9 to 8.5) then the worst that is likely to happen from overdosing (other than truly huge overdosing) is wasting some money and causing some extra precipitation of calcium carbonate on your heater and other parts of the tank. These balanced additives are discussed in more detail at the end of the article.

On the other hand, if you are using independent calcium and alkalinity additives, you must be very careful to not create an imbalance by adding too much of one relative to the other. Support by the use of this calculator can help, but you should rely on frequent testing more than just recommended amounts, to determine how much of these types of additives to put into the tank. Finally, if you are adding large amount of calcium and alkalinity supplements, but just cannot maintain the desired values, you might want to measure the magnesium level in the water.   Magnesium plays an important role in preventing the abiotic precipitation of calcium carbonate, and if it is substantially depleted, you may be experiencing excessive amounts of calcium and alkalinity loss to this route. Magnesium gets the blame far more frequently, in my opinion, than it is likely responsible, but since it is easy to check with a test kit and easy to supplement if necessary, there’s no reason to not see if it is a problem. I’d advise aiming for a natural seawater level of about 1300 ppm.