Low Alk consumption

[carri10]

Hello.
I have a question on alkalinity consumption levels.
(I also have a question on variation of consumption, that is in another thread I have just posted.)
This thread is on the absolute levels of consumption.
I dose 70 mls per day of Randy’s three-part version 1.
For the alkalinity part from Randy’s recipe, if my maths are correct, this 70 mls is equivalent to 0.28dKH per day.

http://reefkeeping.com/issues/2006-02/rhf/index.php

Here it states that the stock is 1,900 meq/L of alkalinity (5,300 dKH).
I add 70/1000 * 1900 meq per day = 133meq into my tank of 1250L
Therefore I add 0.1meq/l/day
As 1meq = 5300/1900dKH (2.79) this means my tank consumes 0.28 dKH / day
(Note, I dose the corresponding amount of Ca and that stays rock solid over a weekly basis at 430ppm).

This seems very low for a tank that is
- 9 months old
-has 11 sps (inc 5 montis)
-11 euphyllia
-7 cyphastrea
-about 20 other corals.

PAR varies from 250 down to 50 from the top rocks to the bottom, provided by 4 x GHL lightbar 3, 12 hour photoperiod, with a 2 hour ramp up morning to 14000k (I think) then about 6 hours full power, then a transition into more blues for the evening.

pH varies from about 8.1 to 8.4
Conductivity is around 34.5
NO3 are 10-20
PO4 is 0.07-0.1
I attached my latest ICP.
I dose Micro E, 20 drops per day.

Corals get fed BRE reef chilli once or twice a week.

Flow is 2 x Maxspect 350, 2 x tunze stream 3+ and a Jebao MLW-30

My corals do grow, but slowly.

Why is my alk consumption so low?
Have I made an idiot of myself with the calculations?
Any opinions welcome!

 

[Randy Holmes-Farley Verified Community Expert]

Calculations are correct.

You have a lot of open water, which effectively raises the water volume and reduces the per gallon alk demand.

 

[carri10]

Thanks for the verification.
So you don’t think that my alk consumption (which I am using as a short time frame proxy for coral growth) suggests they are growing significantly slower than their potential ?

 

[Randy Holmes-Farley Verified Community Expert]

Thanks for the verification.
So you don’t think that my alk consumption (which I am using as a short time frame proxy for coral growth) suggests they are growing significantly slower than their potential ?

They might calcify faster at different conditions, but I don't see a reason to be concerned.