Corrections for Zone 3
Zone 3 problems are a little harder to correct, and are fairly common. It is, in fact, the problem in the real question posed at the beginning of this article (it doesn’t say so there, but the alkalinity was 3.2 meq/L). This problem is typically caused by overdosing alkalinity RELATIVE to calcium, but does not necessarily imply that calcium is either too high or too low (though it is almost always too low). To correct problems in this zone, monitoring of calcium and alkalinity values during correction is especially important.
One more word about this zone before getting to solutions: Many tanks end up here because aquarists are trying to correct pH problems by adding “buffer.” In my opinion, one should not try to correct any pH problem by simply adding an alkalinity supplement. If you are low on alkalinity, it is a fine course of action to raise the alkalinity. But if alkalinity is OK, or even high, adding an alkalinity supplement to alter the pH may simply create a worse problem. Better solutions to pH problems are discussed in this recent article6.
If this problem is extreme (i.e., you are far from the line at the right hand edge of zone 3), then water changes may be the best way to correct to the problem. In most cases, however, water changes aren’t necessary.
I would advise correcting this problem by adding a calcium chloride supplement until you have moved into the target zone (or zones 1 or 2 that you can then treat as described above) as shown in Figure 4. Almost any brand of calcium chloride will do (Kent Turbo Calcium, Kent Liquid Calcium, ESV, etc.). Certain other calcium supplements may also be OK (such as just the calcium component of the two-part calcium and alkalinity additive systems), but you do not want to add any alkalinity. You CANNOT use limewater or a calcium carbonate/carbon dioxide reactor to correct this problem. Any of the balanced calcium and alkalinity additive systems will move you parallel to the line at the edge of the zone, while you want to move over to it, and cross it.
Figure 4. A graph showing how to correct values within zone 3 by adding a calcium additive, such as calcium chloride (the blue arrow).
If calcium is less than 400 ppm, I’d suggest using this handy online calculator7 to determine how much dry calcium chloride is necessary to move back to the target zone. Note that it is a minimum estimate because it does not know how much alkalinity you have, so it cannot know if you are only raising calcium directly (which it calculates) or are also precipitating calcium carbonate (when alkalinity is high this will probably happen, but is typically not a problem other than that it uses up some of what you add).
If the calcium is above 400 ppm in this zone (unlikely, but it does happen), then you can safely either do nothing until it drops and you need to add more calcium, and treat it as suggested in the previous paragraph, or you can add some calcium immediately, move into zone 1, and then just let it drop on its own.
