What causes the PH to drop at night?

[sevise]

I know PH is effected by many variables but the nightly drop is mainly caused by the buildup of CO2. This is then processed by algae and converted to O2 during the day which then increases the PH.

2 days ago I setup my Neptune Systems Aqua controller and even though I haven't calibrated the probe I am still able to monitor the stability of the PH.

My system consists of a 120Gal display, 80gal Sump, 20Gal Fuge w/ chaeto lit 24/7. My lights come on over a 2 hour period between 9AM and 11AM and then turn off from 7PM to 9PM. My PH starts to drop around 830PM in reaction to the MH turning off. However what I find odd is that the PH drops until about 2AM levels off then starts to increase again at about 4AM. I cwuld expect it to drop until the MH come on in the morning. What causes the PH to increase in the middle of the night?

[Randy Holmes-Farley]

I know PH is effected by many variables but the nightly drop is mainly caused by the buildup of CO2. This is then processed by algae and converted to O2 during the day which then increases the PH.


yes, that is correct. :)

I've not tracked the pH that closely to see exactly when it rises and falls so precisely (the pH range is very small in my aquarium), but it is driven by photosynthesis, respiration, and aeration with the home air (and any times additions, like dripping limewater; I assume you do not have any of those)

The CO2 in the home air may follow a slightly different cycle, depending on when you are around breathing, cooking with gas, etc.

Also, organisms do not necessarily photosynthesize and respire at equal rates through the light and dark cycles. They may ramp those activities up and down in response to, or in preparation for, expected light cycles.

[Bojan]

Originally posted by sevise
My system consists of a 120Gal display, 80gal Sump, 20Gal Fuge w/ chaeto lit 24/7. My lights come on over a 2 hour period between 9AM and 11AM and then turn off from 7PM to 9PM. My PH starts to drop around 830PM in reaction to the MH turning off. However what I find odd is that the PH drops until about 2AM levels off then starts to increase again at about 4AM. I cwuld expect it to drop until the MH come on in the morning. What causes the PH to increase in the middle of the night?

It is not usual situation that PH increase in the middle of the night.

Please explain how big is your PH swing
How much your Ph increase between 2 AM and 4AM ?
How many days have you measured this Ph swing and did you have the same graph every day ?
Is it possible that someone open the window at 2 Am ?

[sevise]

I have been monitoring the pH for 3 nights. The first nights drop was 0.08 the second 0.07 and the third 0.05. The first night the fuge was still being lit 24 hours a day. The second I made the change to reverse light it. The third day/night the fuge was completely reverse lit. I graphed up the 3 days inexcel to make comparison easier. I work nights so the house is doesn't change at nights.


I made this graph in excel, the graph start at 9:01 PM just as the last of the lights turns off, the graph end at 9PM. The 3 verticle bars denote when lights come on, the first T5 comes on at 9AM, the MH click on at 10 and 11. The spikes and drop on the pink line are from when I relocated the probe, to my sump. Tick marks a the bottom are on the hour. Breaks in the line are periods when the aquacontroller did not take readings, I'm not sure why this is.
http://cs3.sunyocc.edu/~rassenhe/Images/ph3day.JPG

It looks as if the 24/7 lighting was causing the pH to rise in the middle fo the night. As day 3 on reverse lighting the pH drop slower and until the MH turn on the morning. Also worth noting is that the pH has risen 0.02 each day. Not sure what is causing this.

[Bojan]

Day/night Ph swing 0.08 is very small, so your Ph is very stable.

Based on graph , I conclude that Ph start rising at 7AM. This is time of sunrise. It is possible that Ph start rising as response on sun light, if your aquarium is close to the window.

[sevise]

I'm sorry I put the lines "light on" lines in the wrong spots. I corrected the pic and reuploaded it.

[Randy Holmes-Farley]

I would speculate that perhaps the ambient CO2 level is declining around the tank over the course of the night (after you leave the room and go to bed), allowing pH to rise.

FWIW, those changes are very small. A very small change in something could cause those effects.