alk depletion

[reefman888]

i notice my alk always seem to be depleted much faster than ca. in fact, i dont even need to dose ca. is that normal?
mix reef 95 gal. test kit =hanna

 

[damsels are not mean]

Alk consumption doesn't only go to stony coral skeletons so you can have a lot of biology going on that isn't visible but still moves your alk. Some of it will precipitate and some will be used by coralline growth as well.

 

[Randy Holmes-Farley]

i notice my alk always seem to be depleted much faster than ca. in fact, i dont even need to dose ca. is that normal?
mix reef 95 gal. test kit =hanna

Complicated question.

The most commonly correct explanation is that it is expected to drop much faster from the simple math of calcium carbonate formation.

When Do Calcium and Alkalinity Demand Not Exactly Balance? by Randy Holmes-Farley - Reefkeeping.com


Calcium and Alkalinity Demand: Calcium Carbonate Mathematics

Calcium carbonate formation consumes its two components in an exact 1:1 ratio. In the units used by aquarists, this ratio corresponds to one meq/L (2.8 dKH; 50 ppm CaCO3 equivalents) for every 20 ppm of calcium. Not surprisingly, this is also the ratio of alkalinity to calcium that is supplied when calcium carbonate is dissolved, as in a CaCO3/CO2 reactor. Fortuitously for the aquarist, this is also the ratio supplied when calcium hydroxide is dissolved, as with the use of limewater (kalkwasser).

Apparent Excess Demand for Alkalinity

One of the most common complaints of new aquarists is that their aquaria seem to need more alkalinity than their balanced additive system, such as limewater, is supplying. While there are reasons this may actually be the case over the long term (these will be detailed later in this article), frequently these aquarists are seeing a "chemical mirage" rather than a real excess demand for alkalinity.

One of the interesting features of seawater is that it contains a lot more calcium than alkalinity. By this I mean that if all of the calcium in seawater (420 ppm; 10.5 meq/L) were to be precipitated as calcium carbonate, it would consume 21 meq/L of alkalinity (nearly 10 times as much as is present in natural seawater). In a less drastic scenario, let's say that calcium carbonate is formed from aquarium water starting with an alkalinity of 3 meq/L that it is allowed to drop to 2 meq/L (a 33% drop). How much has the calcium declined? It is a surprise to many people to learn that the calcium would drop by only 20 ppm (5%). Consequently, many aquarists observe that their calcium levels are relatively stable (within their ability to reproducibly test it), but alkalinity can vary up and down substantially. This is exactly what would be expected, given that the aquarium already has such a large reservoir of calcium.

So the first "deviation" from the rule of calcium and alkalinity balance really isn't a deviation at all. If an aquarist is supplying a balanced additive to his aquarium, and calcium seems stable but alkalinity is declining, it may very well be that what is needed is more of the balanced additive, not just alkalinity. This scenario should be assumed as the most likely explanation for most aquarists who should look for more esoteric explanations for alkalinity decline only if calcium RISES substantially while alkalinity falls. Likewise, if alkalinity is rising and calcium seems stable when using a balanced calcium and alkalinity additive system, the most likely explanation is that too much of the additive system is being used.

The real imbalance effects described later in this article take effect slowly, and are manifested over weeks, months and years. This short term "chemical mirage" caused simply by the mathematics of calcium and alkalinity additions can be seen in a single addition. Any effect that develops rapidly over the course of a few days is almost certainly not a true imbalance.

The following scenarios show what can happen to a reef aquarium whose dosage with a balanced additive system does not match its demand. Table 1 shows what can happen when the dosing is inadequate. Alkalinity drops fairly rapidly. After two days, many aquarists might conclude that they need additional alkalinity, when in reality, they need more of both calcium and alkalinity to stabilize the system.

Table 2 shows what happens when too much of a balanced additive is added. After a few days, many aquarists would conclude that alkalinity is rising too much, but that calcium is fairly stable. Again, what is needed is less of the balanced additive, not just less alkalinity.

 

[reefman888]

thks for the detail explanation. if i understand it correctly, the rate ca /alk are being use up are not the same. i need to dose more of both calcium and alkalinity to stabilize the system, even though ca is still high.

 

[Randy Holmes-Farley]

thks for the detail explanation. if i understand it correctly, the rate ca /alk are being use up are not the same. i need to dose more of both calcium and alkalinity to stabilize the system, even though ca is still high.

What exactly are and have been the values?

 

[reefman888]

What exactly are and have been the values?

My alk range from 7 to 8, ca 480 to 525

 

[lubeck]

This was helpful as I’m having a similar situation Regarding alk reduction. I believe it’s due to the large amount of coraline algae on the live rock that I started with and added over 8 corals.

My tank is only 30+ days Old. I do 25% water changes per week. my alk has been around 7.5 but cal stays around 420. I’ve been dosing around 10- 20 ml per day and cal every other day 10-15 ml.
my mag is 1290 and have been dosing 15-20 ml daily.

 

[Randy Holmes-Farley]

My alk range from 7 to 8, ca 480 to 525

If calcium is 500+ ppm, I would hold off on dosing it and just dose alkalinity as needed.

If calcium is lower, but seems stable (say, 400-450 ppm), I'd still dose it is the balanced amount to the alkalinity dosed.

 

[Randy Holmes-Farley]

This was helpful as I’m having a similar situation Regarding alk reduction. I believe it’s due to the large amount of coraline algae on the live rock that I started with and added over 8 corals.

My tank is only 30+ days Old. I do 25% water changes per week. my alk has been around 7.5 but cal stays around 420. I’ve been dosing around 10- 20 ml per day and cal every other day 10-15 ml.
my mag is 1290 and have been dosing 15-20 ml daily.

Same advice as above, and for you, dose both calcium and alkalinity.

Coraline does use a lot of calcium and alkalinity, and cannot use alk without using calcium.