View Full Version : lower alk = lower demand?

09/22/2012, 09:04 PM
Does a 2/3 drop in alk demand from an alk of 9 to 7.5 sound normal?

I have a few frags in a 40 breeder sharing the sump with my coral free 90 gallon (total of 125 gallons of water after displacement). I have been running my alk at 9 dKH (reading before dosing). My demand was low at 0.75 dKH per day.

I started vodka/vinegar dosing and decided to play it safe and lower my alk to between 7 and 8. I stopped dosing and have watched the alk lower. As it gets lower, the demand is less. I expected this because I seem to remember that higher alk means faster growth.

However, the demand now that alk is below 8 dKH seems to be about 0.25 dKH per day. I was wondering if this drastic a drop is normal. I have not starting dosing again to see if I can confirm the new demand level, but figured I would post here in he mean time.

I tested with both Salifert and Hanna Checker. I don't know if it matters, but my calcium and Mg are a little high because of an old test kit that has since been replaced. So of course I have not been dosing that for a little bit even before dropping my alk. Of course, with the lower alk demand, the calcium is dropping even slower.

09/22/2012, 09:07 PM
That sounds about normal to me. As Alkalinity (Calcium too) becomes mores scarce in the water column, growth of corals and coralline slows, causing their demand to decrease.

09/22/2012, 09:11 PM
That is a fairly drastic drop. I might get a second opinion on the alkalinity test kit to make sure it's not reading a bit high, but I wouldn't worry much.

09/22/2012, 09:52 PM
I am not understanding this quite right. Wouldn't a drop in alk from 9 to 7.5 mean and increase in demand for carbonate by the corals?
I also do not see an alk level of 7.5 a limiting factor on coral growth as it is close to normal sea water levels

09/22/2012, 11:02 PM
There could be something else going on that is slowing consumption beyond the slower growth with lower alk. While seawater is around 7dkh, calcification( biotic precipitation of calcium carbonate increases when alk is higher and there is adequate calcium and magnesium .
How do your corals look?
The increased levels of dissolved organic material from the dosing may be slowing abiotic and biotic precipitation of calcium carbonate for example thus reducing consumption too.

09/22/2012, 11:31 PM
Higher alkalinity levels can encourage more growth, up to a fairly high alkalinity level, above 11 dKH. Of course, other limiting factors might influence any given tank. So I'm not surprised that the growth is dropping at least somewhat with the lower dKH level. That said, that drop is rather large, and there might be other factors at play. I'd probably measure carefully for a week, the same time every day, and see how that goes.