Trying to understand why my alkalinity is dropping

Dawgvet · May 1, 2018

History:
9 1/2 year old, mainly SPS tank. 75 gal display/20 gal sump/fuge (approx total water volume 60 gal).
Weekly 4 gal changes every Tuesday, Red Sea Coral Pro salt mix.
Weekly parameter measurements every Saturday.
Salifert and Hanna test kits used. All within exp date.

Daily dosing through BRS peristaltic pumps:
BRS Calcium Chloride: 19.8 mls/dose x 4 ( 79.2 mls/day)
RHF Recipe #1, part 2: 8.4mls/dose x 4 (33.6 mls/day)
BRS 2 part Mg Cl/SO2-: 21 mls/dose x 2 (41 mls/day) * dosed every 12 hours.*
pH monitoring with PinPoint monitor, 2 month old probe, calibrated every 2 weeks.

Last measurements:

Magnesium dosing was discontinued April 1.
Avg pH in the morning 7.9/ afternoon 8.1

**This morning's pH 7.8 and Alkalinity 7.0 dKH Salifert/6.9 Hanna.**

The only difference in the past week is that I did start micro/nano bubbling ( I know, I know Randy)
The air pump is outside (16 L/min) with dual lines going to wood air stones and they are on for 10 hrs at night. (9 pm to 7 am)
Skimmer air intake also outside.
So my confusion is, if I'm pumping more outside oxygen at night time, there should be less CO2, and therefore, my pH should be more stable (which it is) but shouldn't my alkalinity be more stable and not suddenly start dropping? My alkalinity has dropped around 1.4 dKH since starting bubbles.
Any ideas why the drop?
Thank you in advance.

Randy Holmes-Farley · May 2, 2018

If you reduced the low pH attained at night, that may increase demand. Growing corals will need more. Maybe the bubbling helps them grow faster, etc.

The change looks small to me. I'd just dose a bit more.

Dawgvet · May 2, 2018

Thank you,
I was just wondering what the direct correlation (if any) between night time bubbling and decreasing alkalinity could be.
I will increase dosing and monitor.

addendum: Randy, at what point is a parameter change considered a "swing". You always read about stability and avoiding parameter swings but I don't know if I've ever read, using alkalinity as an example, if a 1 dKH change in a week is ok but a 2 dK is considered too much of a change and is a "swing". Is it more the see-sawing back and forth of a parameter or just a single directional change? I realize that this may not be an easy question to answer.

Dawgvet · May 2, 2018

double post

Randy Holmes-Farley · May 2, 2018

Dawgvet said:
Thank you,
I was just wondering what the direct correlation (if any) between night time bubbling and decreasing alkalinity could be.
I will increase dosing and monitor.

addendum: Randy, at what point is a parameter change considered a "swing". You always read about stability and avoiding parameter swings but I don't know if I've ever read, using alkalinity as an example, if a 1 dKH change in a week is ok but a 2 dK is considered too much of a change and is a "swing". Is it more the see-sawing back and forth of a parameter or just a single directional change? I realize that this may not be an easy question to answer.

There's no "direct" cause. Indirect changes in pH or biological effects of bubbles de-sliming corals might have an impact.

I think we anything close to lack complete data on what changes in alkalinity over what time periods are an issue, and even, for that matter, how one defines what an issue is.

The change in alkalinity from steady at 9 dKH for a month then instantly to 7 dKH for a month may be undesirable. If its swing from 7 dKH to 9 dKH each 5 minutes, and back again, I suspect that might not be an issue.

IMO, the issue likely relates to corals adapting their alkalinity uptake systems to match a certain availability. Then if that availability changes, they may suddenly get too much or too little. It will take time (hours? days?) for corals to do that so very fast changes and very slow changes are both likely less of an issue then changes in alk on a slightly faster time frame than corals can adapt.