Is this true ?

[I never finish anythi]

This is something that's got posted on a what's app group I'm in !

Is this how it works as it's above my brain cell capability!

Yes, it can as nitrifying bacteria will use CO2 which they take from CaCO3 and that will deplete alkalinity (as the result is Ca + O and the carbonate no longer exists). However if denitrification is allowed to complete denitrifyers will consume the O2 from NO3 and will release CO2 which then will bind the Calcium and Oxigen to for CaCO3, I.e. Calcium carbonate resulting in an increase of alkalinity. If this happens the typically there is a net 0 in terms of alkalinity shift.

The problem is when the cycle is interrupted by something (e.g. water changes), then alkalinity can go up or down without the dosing of supplements

 

[I never finish anythi]

Can any one dumb it down for me please ?

 

[Dipi]

There is a section on this article from Randy that explains what happens. https://www.reef2reef.com/ams/when-do-calcium-and-alkalinity-demand-not-exactly-balance.954/

 

[I never finish anythi]

There is a section on this article from Randy that explains what happens. https://www.reef2reef.com/ams/when-do-calcium-and-alkalinity-demand-not-exactly-balance.954/

Thank you .

 

[Randy Holmes-Farley]

The basic conclusion is true:

rising nitrate depletes alk
falling nitrate adds alk
dosing and consumption of nitrate adds alk

but I don't think the explanation as written in the first post is true since the processes do not really involve CO2.

The link given above explains it in great detail, and I'm happy to explain it further as needed:

Alkalinity Decline in the Nitrogen Cycle

One of the best known chemical cycles in aquaria is the nitrogen cycle. In it, ammonia excreted by fish and other organisms is converted into nitrate. This conversion produces acid, H+ (or uses alkalinity depending on how one chooses to look at it), as shown in equation 1:


(1) NH3 + 2O2 --> NO3- + H+ + H2O

For each ammonia molecule converted into nitrate, one hydrogen ion (H+) is produced. If nitrate is allowed to accumulate to 50 ppm, the addition of this acid will deplete 2.3 dKH of alkalinity.

However, the news is not all bad. When this nitrate proceeds further along the nitrogen cycle, the depleted alkalinity is returned in exactly the amount lost. For example, if the nitrate is allowed to be converted into N2 in a sand bed, one of the products is bicarbonate, as shown in equation 2 (below) for the breakdown of glucose and nitrate under typical hypoxic conditions as might happen in a deep sand bed:

(2) 4NO3- + 5/6 C6H12O6 (glucose) + 4H2O --> 2 N2 + 7H2O + 4HCO3- + CO2

In equation 2 we see that exactly one bicarbonate ion is produced for each nitrate ion consumed. Consequently, the alkalinity gain is 2.3 dKH for every 50 ppm of nitrate consumed.

Likewise, equation 3 (below) shows the uptake of nitrate and CO2 into macroalgae to form typical organic molecules:

(3) 122 CO2 + 122 H2O + 16 NO3- --> C106H260O106N16 + 138 O2 + 16 HCO3-

Again, one bicarbonate ion is produced for each nitrate ion consumed.

It turns out that as long as the nitrate concentration is stable, regardless of its actual value, there is no ongoing net depletion of alkalinity. Of course, alkalinity was depleted to reach that value, but once it stabilizes, there is no continuing alkalinity depletion because the export processes described above are exactly balancing the depletion from nitrification (the conversion of ammonia to nitrate). This also applies to all types of organic carbon dosing schemes, unless the organic added actually provides alkalinity when it is metabolized, such as with formate in All For Reef or acetate in Salifert All in One.

There are, however, circumstances where the alkalinity is lost in the conversion of ammonia to nitrate, and is never returned. The most likely scenario to be important in reef aquaria is when nitrate is removed through water changes. In that case, each water change takes out some nitrate, and if the system produces nitrate to get back to some stable level, the alkalinity again becomes depleted.

If, for example, nitrate averages 50 ppm at each water change, then over the course of a year with 10 water changes of 20% each, the alkalinity will be depleted by 4.5 dKH over the course of that entire time period. This process is one of the primary reasons that fish-only aquaria that often export nitrate in water changes need occasional buffer additions to replace that depleted alkalinity, even if nitrate is the same after each water change.

While the magnitude of the depletion described in the paragraph above due to water changes is fairly easy to understand, it also can be converted into units that clarify the imbalance. The impact of alkalinity depletion on the calcium and alkalinity demand balance depends, of course, on the amount of calcium and alkalinity added (and consumed) over the course of that same year.

For a typical reef aquarium (for example, an aquarium using a daily addition of saturated limewater equal to 2% of the tank's volume), the amount of alkalinity added during the course of a year is very large: 834 dKH. Likewise, the amount of calcium added is 5,957 ppm Ca++. If that extra 4.5 dKH of alkalinity lost by water changes containing nitrate is added to create a slightly larger demand of 838.8 dKH over the course of a year, the new ratio for the alkalinity to calcium consumed is changed very little: from 20.0 ppm calcium for each 2.8 dKH to 19.9 ppm for each 2.8 dKH of alkalinity. Consequently, while the effect of nitrate production on alkalinity is enough to be noticed over the course of a year, it is substantially smaller than some of the other effects discussed in this article, and is unimportant for aquaria that maintain low nitrate levels.


Effects of Nitrate Dosing

Taking equations 2 and 3 from above, representing nitrate being consumed by either denitrification (#2) or by incorporation into tissues (#3)

(2) 4NO3- + 5/6 C6H12O6 (glucose) + 4H2O --> 2 N2 + 7H2O + 4HCO3- + CO2
(3) 122 CO2 + 122 H2O + 16 NO3- --> C106H260O106N16 + 138 O2 + 16 HCO3-

we see that if we are dosing nitrate and it is consumed, there is generation of alkalinity (as bicarbonate in these equations, HCO3-). In both cases, the addition is 2.3 dKH for each 50 ppm of nitrate dosed and consumed. That amount is significant enough that in a low alkalinity demand reef aquarium, alkalinity may actually rise over time without any standard alkalinity additives being used. In all aquaria, it is sufficient to throw off the standard ratio of alkalinity to calcium consumed.

Figure 7. Bubble tip (E. quad.) anemones in the aquarium of Reef2Reef member Hitman. While anemones themselves are not net consumers of alkalinity or calcium, they may use alkalinity in the water as a source of CO2 for photosynthesis. In this aquarium, nitrate was being dosed, which will tend to supplement alkalinity as the nitrate is consumed.

 

[I never finish anythi]

The basic conclusion is true:

rising nitrate depletes alk
falling nitrate adds alk
dosing and consumption of nitrate adds alk

but I don't think the explanation as written in the first post is true since the processes do not really involve CO2.

The link given above explains it in great detail, and I'm happy to explain it further as needed:

Alkalinity Decline in the Nitrogen Cycle

One of the best known chemical cycles in aquaria is the nitrogen cycle. In it, ammonia excreted by fish and other organisms is converted into nitrate. This conversion produces acid, H+ (or uses alkalinity depending on how one chooses to look at it), as shown in equation 1:


(1) NH3 + 2O2 --> NO3- + H+ + H2O

For each ammonia molecule converted into nitrate, one hydrogen ion (H+) is produced. If nitrate is allowed to accumulate to 50 ppm, the addition of this acid will deplete 2.3 dKH of alkalinity.

However, the news is not all bad. When this nitrate proceeds further along the nitrogen cycle, the depleted alkalinity is returned in exactly the amount lost. For example, if the nitrate is allowed to be converted into N2 in a sand bed, one of the products is bicarbonate, as shown in equation 2 (below) for the breakdown of glucose and nitrate under typical hypoxic conditions as might happen in a deep sand bed:

(2) 4NO3- + 5/6 C6H12O6 (glucose) + 4H2O --> 2 N2 + 7H2O + 4HCO3- + CO2

In equation 2 we see that exactly one bicarbonate ion is produced for each nitrate ion consumed. Consequently, the alkalinity gain is 2.3 dKH for every 50 ppm of nitrate consumed.

Likewise, equation 3 (below) shows the uptake of nitrate and CO2 into macroalgae to form typical organic molecules:

(3) 122 CO2 + 122 H2O + 16 NO3- --> C106H260O106N16 + 138 O2 + 16 HCO3-

Again, one bicarbonate ion is produced for each nitrate ion consumed.

It turns out that as long as the nitrate concentration is stable, regardless of its actual value, there is no ongoing net depletion of alkalinity. Of course, alkalinity was depleted to reach that value, but once it stabilizes, there is no continuing alkalinity depletion because the export processes described above are exactly balancing the depletion from nitrification (the conversion of ammonia to nitrate). This also applies to all types of organic carbon dosing schemes, unless the organic added actually provides alkalinity when it is metabolized, such as with formate in All For Reef or acetate in Salifert All in One.

There are, however, circumstances where the alkalinity is lost in the conversion of ammonia to nitrate, and is never returned. The most likely scenario to be important in reef aquaria is when nitrate is removed through water changes. In that case, each water change takes out some nitrate, and if the system produces nitrate to get back to some stable level, the alkalinity again becomes depleted.

If, for example, nitrate averages 50 ppm at each water change, then over the course of a year with 10 water changes of 20% each, the alkalinity will be depleted by 4.5 dKH over the course of that entire time period. This process is one of the primary reasons that fish-only aquaria that often export nitrate in water changes need occasional buffer additions to replace that depleted alkalinity, even if nitrate is the same after each water change.

While the magnitude of the depletion described in the paragraph above due to water changes is fairly easy to understand, it also can be converted into units that clarify the imbalance. The impact of alkalinity depletion on the calcium and alkalinity demand balance depends, of course, on the amount of calcium and alkalinity added (and consumed) over the course of that same year.

For a typical reef aquarium (for example, an aquarium using a daily addition of saturated limewater equal to 2% of the tank's volume), the amount of alkalinity added during the course of a year is very large: 834 dKH. Likewise, the amount of calcium added is 5,957 ppm Ca++. If that extra 4.5 dKH of alkalinity lost by water changes containing nitrate is added to create a slightly larger demand of 838.8 dKH over the course of a year, the new ratio for the alkalinity to calcium consumed is changed very little: from 20.0 ppm calcium for each 2.8 dKH to 19.9 ppm for each 2.8 dKH of alkalinity. Consequently, while the effect of nitrate production on alkalinity is enough to be noticed over the course of a year, it is substantially smaller than some of the other effects discussed in this article, and is unimportant for aquaria that maintain low nitrate levels.


Effects of Nitrate Dosing

Taking equations 2 and 3 from above, representing nitrate being consumed by either denitrification (#2) or by incorporation into tissues (#3)

(2) 4NO3- + 5/6 C6H12O6 (glucose) + 4H2O --> 2 N2 + 7H2O + 4HCO3- + CO2
(3) 122 CO2 + 122 H2O + 16 NO3- --> C106H260O106N16 + 138 O2 + 16 HCO3-

we see that if we are dosing nitrate and it is consumed, there is generation of alkalinity (as bicarbonate in these equations, HCO3-). In both cases, the addition is 2.3 dKH for each 50 ppm of nitrate dosed and consumed. That amount is significant enough that in a low alkalinity demand reef aquarium, alkalinity may actually rise over time without any standard alkalinity additives being used. In all aquaria, it is sufficient to throw off the standard ratio of alkalinity to calcium consumed.

Figure 7. Bubble tip (E. quad.) anemones in the aquarium of Reef2Reef member Hitman. While anemones themselves are not net consumers of alkalinity or calcium, they may use alkalinity in the water as a source of CO2 for photosynthesis. In this aquarium, nitrate was being dosed, which will tend to supplement alkalinity as the nitrate is consumed.

Thank you Randy Appreciate it .