What determines the diurnal PH cycle in a reef tank?

[Dave-T]

I thought I understood this, but when I look at the PH in my tank I realize I can't explain what I'm seeing. What I thought was going on was that the photosynthetic organisms in my tank (corals, algae, ...) were absorbing C02 during the light period, driving up the PH. And it would go down at night, outside the photosynthetic period. When I look at the PH swing in my tank, I do see a pretty consistent daily minimum at about 6AM, and a max at about 6PM. But this isn't very close to the timing the light period in my tank. My lights come on at 8:30 AM, have a 3 hour ramp up to the peak lighting, then stay at peak until 7:30PM, when they start a 2 hour ramp down.

So my PH hits the minimum and starts climbing 2.5 hours before the lights come on. Can anyone explain this? Is this normal?

I thought maybe it was due to C02 levels in the air in my house, but I have a C02 monitor and if I look at a graph of the readings from that, it's pretty consistent through the day, and fairly low - for the last 24 hours it hit a minimum of 433 and a max of 566ppm, and there isn't any kind of sine wave to the graph of that data.

 

[Dan_P]

I thought I understood this, but when I look at the PH in my tank I realize I can't explain what I'm seeing. What I thought was going on was that the photosynthetic organisms in my tank (corals, algae, ...) were absorbing C02 during the light period, driving up the PH. And it would go down at night, outside the photosynthetic period. When I look at the PH swing in my tank, I do see a pretty consistent daily minimum at about 6AM, and a max at about 6PM. But this isn't very close to the timing the light period in my tank. My lights come on at 8:30 AM, have a 3 hour ramp up to the peak lighting, then stay at peak until 7:30PM, when they start a 2 hour ramp down.

So my PH hits the minimum and starts climbing 2.5 hours before the lights come on. Can anyone explain this? Is this normal?

I thought maybe it was due to C02 levels in the air in my house, but I have a C02 monitor and if I look at a graph of the readings from that, it's pretty consistent through the day, and fairly low - for the last 24 hours it hit a minimum of 433 and a max of 566ppm, and there isn't any kind of sine wave to the graph of that data.

Is the aquarium water temperature constant?

Are you dosing anything?

 

[Randy Holmes-Farley]

It is not just photosynthesis that drives the changes, but respiration (production of CO2). I’m not sure exactly how that changes over a 24 h cycle.

 

[Dave-T]

Is the aquarium water temperature constant?

Are you dosing anything?

I am dosing kalc, mostly at night. That would flatten out the amplitude of the pH sign wave, but wouldn't change what the times are of the min and max.

 

[Dave-T]

And yes, my water temperature is constant.

 

[Randy Holmes-Farley]

I am dosing kalc, mostly at night. That would flatten out the amplitude of the pH sign wave, but wouldn't change what the times are of the min and max.

The pH is a balance between lots of different processes that add and remove alk and CO2, and I think it is hopeless to try to explain minor details of exactly which processes are most active at which times.

 

[Dave-T]

What's most interesting to me, which I thought I said above but now see I didn't - is that my PH cycle seems to pretty closely follow the cycle of daylight outside, which is quite different from the light cycle in my tank. As if the biological clock of the organisms in my tank were synchronized with the sun, rather than the artificial cycle imposed by my light period. There is light coming in the room from outside, but it's not like direct sunlight is shining on my tank.

What I'd like to know is what other people see. Does your PH cycle more closely correspond to your tank lighting schedule? Or is it more typical for it to coincide with the day/night cycle outside?

 

[Randy Holmes-Farley]

I would not have been able to distinguish in my tank since they were not significantly different.

What times of day exactly do you dose the kalkwasser? Alk may peak right after it finishes, and that higher alk leads to higher pH at the same CO2.

 

[Dave-T]

I would not have been able to distinguish in my tank since they were not significantly different.

What times of day exactly do you dose the kalkwasser? Alk may peak right after it finishes, and that higher alk leads to higher pH at the same CO2.

I dose 80% of the kalk at night, 20% during the day. And the nightly dosing rate is exactly during the times when my PH is in the lower range of the sine wave. But that's because I picked my kalk dosing times based on what PH swing I had before I ever started dosing kalk. The kalk didn't change the timing of the ph swing, it just raised the average PH and flattened out the curve.

 

[jda]

When my furnace runs, it mixes the air in the home and can move higher co2 air from the bedrooms and living rooms to the areas with my tank (and co2 monitor). It is not a lot, but airborne co2 does tend to go up a bit in the early AM in the area with my tank since the furnace has been running a bit in the early AM lately with fall coming.

This probably not all of the explanation, but it could be some.