Alkalinity drops whether I dose soda ash or not

[Fritz05]

My alkalinity drops by about 1 dKH a day (65g mixed reef, bi-weekly 10% water changes with Instant Ocean, lots of coralline, incl. back and side walls completely covered). This happens whether I dose or not!

I used the BRSTV method of stopping any dosing, measuring alk on day 1 (8 dKH), and measuring it again on day 2 at the same time with a Hanna checker, showing 7 dKH. So I dosed back to 8 dKH and measured again after 24 hours, showing 6.7 dKH. I did it a third time showing a similar result.

I determined a daily dose of liquid soda ash of 60 ml was needed, using the BRS calculator. After dosing back to 8 dKH I set the dose to 60 ml in 12 daily 5 ml increments using a dosing pump. I checked that the pump works and it dispenses 5 ml each time. I do not see precipitation during dosing.

Today I measured at the same hour and the alk is at 7.2! I used Hanna reagent from the same bottle for all measurements.

FYI: I stopped dosing Ca during this period, as I wanted to focus on alk. It did drop from about 420 to 360.

I have not changed anything in my tank in months.

Any ideas?

 

[Randy Holmes-Farley Verified Community Expert]

If you do not appear to be getting calcium carbonate precipitation (hardening sand, seizing up pump impellers, etc) then just up the dose. The method of watching a drop is not the best because demand depends on pH and the absolute alk, both of which decline during such a test.

 

[Mortgageman]

What if you are getting precip in sump? I am having a similar issue. I have 90 gall with lots of coralline. Only 4 frags and 5 zoa colonies. My drop in kh is 1.63 dkh in 22 hours. Seems way too much for what I have. I am using BRS 2part that’s 141 ml per day and I have precip. I am also not sure of the fix here

 

[Randy Holmes-Farley Verified Community Expert]

A soft coral tank can use 2 dKH per day if there is coralline growth.

When you say you have precipitate, can you describe what that means? A transient ppt that redissolves, or substantial layers stuck to the sump sides?

Switching to the bicarbonate version will greatly reduce precipitation.

 

[Fritz05]

FYI, I upped the dose as Randy suggested and now have relatively stable alk. I had to overcome the impression that my tank should not need as high a dose as it actually does. I am at 85ml BRS Alk per day now for my 65g mixed reef.