Reef Chemistry Question of the Day #244 Imbalanced Dosing and Salt Mixes

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #244

One of the primary reasons that reefers may needed to dose an unbalanced ratio of calcium to alkalinity in a reef aquarium is to counter the effects of a water change that does not match the aquarium.

Suppose that your target levels in your reef aquarium for calcium and alkalinity are 430 ppm and 8 dKH.

Which of the follow salt mix parameters, when used for water changes, will cause a substantial imbalance in the above tank such that more alkalinity than calcium is needed when dosing?

A. 5.9 dKH, 416 ppm calcium
B. 6.6 dKH, 421 ppm calcium
C. 10.8 dKH, 449 ppm calcium
D. 12.2 dKH, 492 ppm calcium
E. All of the above

Good luck!















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[MnFish1]

Changing DKH to PPM, and calculating the ratio of Ca/Alkalinity of the example Tank is 3.00

The ratios (Ca/Alk). for the others are A. 3.9 B. 3.6 C. 2.3 D. 2.3

The Alk of A and B are relatively lower than the Calcium for Choices A and B - So I would assume that the dosing for A and B would be higher.
Conversely, I wonder of C and D with the very high Alk and CA - might cause more precipitation - and thus also a requirement for increased dosing of ALK -

So I guess: E

 

[Randy Holmes-Farley]

Conversely, I wonder of C and D with the very high Alk and CA - might cause more precipitation - and thus also a requirement for increased dosing of ALK -

So I guess: E

It might, but there's no need to invoke precipitation for the answer (and wouldn't change it anway).

 

[Shon]

Hmm.. "Substantial Imbalance", "more alkalinity than calcium is needed when dosing?"..

A. 2.1dkh/14ppm
B. 1.4dkh/9ppm
C. +2.8dkh/+19ppm
D. +4.2dkh/+62ppm
E. AOA

Hmm.. A?

 

[MnFish1]

It might, but there's no need to invoke precipitation for the answer (and wouldn't change it anway).

OK - Then its A and B

 

[Randy Holmes-Farley]

And the answer is... D

Suppose that your target levels in your reef aquarium for calcium and alkalinity are 430 ppm and 8 dKH.

Which of the follow salt mix parameters, when used for water changes, will cause a substantial imbalance in the above tank such that more alkalinity than calcium is needed when dosing?

A. 5.9 dKH, 416 ppm calcium
B. 6.6 dKH, 421 ppm calcium
C. 10.8 dKH, 449 ppm calcium
D. 12.2 dKH, 492 ppm calcium
E. All of the above

Let's explore all of the answers.

Before I discuss each option, I'll note that a balanced additives will add about 18-20 pppm of calcium for each 1 meq/L (2.8 dKH) of alkalinity, or about 6.4 to 7.1 ppm per dKH. The exact ratio depends on how much magnesium and strontium are being incorporated in place of calcium in the aragonite structures.


A. 5.9 dKH, 416 ppm calcium.

That puts it 2.1 dKH and 14 ppm calcium below the tank water. Both calcium and alkalinity will be needed, and in a ratio of 14 ppm per 2.1 dKH or 6.7 ppm per dKH. About a balanced ratio of increased dosing. So no mismatched dosing is required due to this ratio in the salt mix. Equal and more dosing of each is needed to offset the water change.

B. 6.6 dKH, 421 ppm calcium

That puts it 1.4 dKH and 9 ppm calcium below the tank water. Both calcium and alkalinity will be needed, and in a ratio of 9 ppm per 1.4 dKH or 6.4 ppm per dKH. About a balanced ratio of increased dosing. So no mismatched dosing is required due to this ratio in the salt mix. Equal and more dosing of each is needed to offset the water change.

C. 10.8 dKH, 449 ppm calcium

That puts it 2.8 dKH and 19 ppm calcium above the tank water. Both calcium and alkalinity will be excess, and in a ratio of 19 ppm per 2.8 dKH or 6.8 ppm per dKH. About a balanced ratio of excess. So no mismatched dosing is required due to this ratio in the salt mix. Equal and less dosing of each. Equal and less dosing of each (going forward) is needed to offset the water change.

D. 12.2 dKH, 492 ppm calcium

That puts it 4.2 dKH and 62 ppm calcium above the tank water. Both calcium and alkalinity will be excess, and in a ratio of 62 ppm per 4.2 dKH or 14.8 ppm per dKH. That is NOT a balanced ratio of excess. It adds too much calcium per alkalinity, even though it may look, at first glance, to be higher in alkalinity, it is really the calcium that is most excessive (relative to the target levels). Mismatched dosing will required (going forward) due to this ratio in the salt mix. Less dosing of the alk part will be needed than calcium to offset the water change.


Happy Reefing.