Jestersix

For people directing outdoor air to your skimmer ... did you drill a hole in your house?

PizzaOven

Supporting Member
I wouldn't expect DO to ever exceed 100% (and by 100% that's 100% of air saturation or 21% oxygen of the total gases at sea level) except in extreme situations where you have a massive amount of photosynthesis. We're sparging in room air which will equilibrate with the tank pretty quickly in a skimmer based system. Even if you remove excess CO2 from animals exchanging via their gills in the tank though heavy off gassing, you won't drop below atmospheric ~0.25-3 mmHg (your blood is around 35-45 mmHg in comparison) unless you scrub CO2. So you'd either have to pipe in pure oxygen to reach hyperoxic conditions (which wouldn't affect pH as much as reducing CO2 due to the buffering capacity of the bicarbonate). Correct me if I'm wrong here of course!

The paper is claiming that higher pH is more important for light calcification while 100% DO is more important than pH for dark calcification. My guess is that during the day the zooxanthelle are creating excess oxygen inside the coral tissue, whereas at night, there's more oxygen in the water than coral tissue as internal photosynthesis stops, leaving them more sensitive to hypoxic conditions.

In short, keep the system well aerated, reduce ambient CO2, and keeping temperature lower may help as well because lower temperature water has higher solubility for gases.

What's interesting is that a reference from this paper claims:

"coral reef waters during daytime is always accompanied by hyperoxia"

They measure DO between 27% (1.67 mg/L) to 241% (14.19 mg/L) over a full day-night cycle on the coral reef.

Of course, our aquariums are not direct analogs to the ocean. Is the scale of photosynthesis in the ocean enough to create hyperoxic conditions, while in our home aquariums it is not?
 

PizzaOven

Supporting Member
We're sparging in room air which will equilibrate with the tank pretty quickly in a skimmer based system.
I think I see your point here, in a skimmer based system, excess DO in the water column will equilibrate with the air being pumped in, hence why we shouldn't see hyperoxic conditions?
 

JVU

President
BOD
Dissolved oxygen is almost always super high in a typical reef aquarium regardless of skimmer/no skimmer. Everyone should test this themselves if given the chance. I have a Milwaukee DO meter I don’t use any more that I’d be happy to donate to BAR. You can sorta use DO as a proxy for how much gas exchange you’re getting, unless you want to buy a TDG meter ($$$).

I guess my point here is that the question about whether a skimmer or surface agitation exchanges more gas is kinda moot; what’s more important IMO is making sure the air around that water/air exchange is “fresh”, as in low in CO2. It’s cool you guys have a CO2 meter.

You can also confirm whether your gas exchange is good by testing the pH of your tank, removing some water into a beaker and bubble air into it like crazy, and continue to test pH in that beaker (do it in the same room at the same temp). If the pH stays the same, you’re good. Just a bit more involved process.
Having a DO meter for the club members to borrow would be awesome! We have the BARCode system to track our equipment, which helps to keep it accounted for.

Thanks for the insight Matt.

I believe seeing normal O2 levels in tank water does not necessarily mean we should assume that all gasses are exchanged at ambient equilibrium in our tanks. The amounts of the gasses (partial pressures) and the increases and decreases due to biology and chemistry in our tanks make a difference. For example, oxygen in the air is about 210,000 ppm whereas CO2 is about 420 ppm (outside), a 500-fold difference in concentration and partial pressure. Concentrations in the tank water don’t follow that ratio exactly, but a huge differential persists. Increasing either by the equivalent 1000 ppm in air would make an imperceptible difference to measured O2 level, but would make a big difference pH for the CO2 level change. Having animals (including coral) in our tank using up O2 and producing the same amount of CO2 with metabolism likewise can make a big difference in increased CO2 (decreasing pH) even though the O2 reading is not perceptibly changed.

I have tested tank water pH vs ambient CO2 level in the tank (with Alk etc accounted for) and the pH in the tank is always lower than expected when the lights are off. When photosynthesis is in full swing it can normalize it, but if we are focussing on equilibration with ambient air this is kind of cheating since photosynthesis is consuming the CO2. More simply, all of us with an Apex can see that there is a big swing in pH between night and day even when our flow/skimmer/etc are all the same, and even when the ambient CO2 is basically the same. The more animal tissue in the tank, the bigger the swing. This tells us that our tanks are not really in equilibrium with ambient CO2, but rather that when the lights are off, our CO2 level in our tank is much higher than would be expected based on the amount in our air. If we truly had good near-100% gas exchange with ambient air for CO2, there would be no pH swing. I tried increasing the aeration with an overpowered air pump and a bunch of air stones to reduce this effect, and it didn’t make much difference in the pH.
 

Matt_Wandell

Supporting Member
Honorary Supporting Member
Having a DO meter for the club members to borrow would be awesome! We have the BARCode system to track our equipment, which helps to keep it accounted for.

Thanks for the insight Matt.

I believe seeing normal O2 levels in tank water does not necessarily mean we should assume that all gasses are exchanged at ambient equilibrium in our tanks. The amounts of the gasses (partial pressures) and the increases and decreases due to biology and chemistry in our tanks make a difference. For example, oxygen in the air is about 210,000 ppm whereas CO2 is about 420 ppm (outside), a 500-fold difference in concentration and partial pressure. Concentrations in the tank water don’t follow that ratio exactly, but a huge differential persists. Increasing either by the equivalent 1000 ppm in air would make an imperceptible difference to measured O2 level, but would make a big difference pH for the CO2 level change. Having animals (including coral) in our tank using up O2 and producing the same amount of CO2 with metabolism likewise can make a big difference in increased CO2 (decreasing pH) even though the O2 reading is not perceptibly changed.

I have tested tank water pH vs ambient CO2 level in the tank (with Alk etc accounted for) and the pH in the tank is always lower than expected when the lights are off. When photosynthesis is in full swing it can normalize it, but if we are focussing on equilibration with ambient air this is kind of cheating since photosynthesis is consuming the CO2. More simply, all of us with an Apex can see that there is a big swing in pH between night and day even when our flow/skimmer/etc are all the same, and even when the ambient CO2 is basically the same. The more animal tissue in the tank, the bigger the swing. This tells us that our tanks are not really in equilibrium with ambient CO2, but rather that when the lights are off, our CO2 level in our tank is much higher than would be expected based on the amount in our air. If we truly had good near-100% gas exchange with ambient air for CO2, there would be no pH swing. I tried increasing the aeration with an overpowered air pump and a bunch of air stones to reduce this effect, and it didn’t make much difference in the pH.
Hey John, you’re totally right. Photosynthesis changes all this. I should have said it’s best to do this first thing in the morning before lights come on. It’s fair to say that if your DO is 100% or near it at that time then your gas exchange is good.
 

Matt_Wandell

Supporting Member
Honorary Supporting Member
Having a DO meter for the club members to borrow would be awesome! We have the BARCode system to track our equipment, which helps to keep it accounted for.

Thanks for the insight Matt.

I believe seeing normal O2 levels in tank water does not necessarily mean we should assume that all gasses are exchanged at ambient equilibrium in our tanks. The amounts of the gasses (partial pressures) and the increases and decreases due to biology and chemistry in our tanks make a difference. For example, oxygen in the air is about 210,000 ppm whereas CO2 is about 420 ppm (outside), a 500-fold difference in concentration and partial pressure. Concentrations in the tank water don’t follow that ratio exactly, but a huge differential persists. Increasing either by the equivalent 1000 ppm in air would make an imperceptible difference to measured O2 level, but would make a big difference pH for the CO2 level change. Having animals (including coral) in our tank using up O2 and producing the same amount of CO2 with metabolism likewise can make a big difference in increased CO2 (decreasing pH) even though the O2 reading is not perceptibly changed.

I have tested tank water pH vs ambient CO2 level in the tank (with Alk etc accounted for) and the pH in the tank is always lower than expected when the lights are off. When photosynthesis is in full swing it can normalize it, but if we are focussing on equilibration with ambient air this is kind of cheating since photosynthesis is consuming the CO2. More simply, all of us with an Apex can see that there is a big swing in pH between night and day even when our flow/skimmer/etc are all the same, and even when the ambient CO2 is basically the same. The more animal tissue in the tank, the bigger the swing. This tells us that our tanks are not really in equilibrium with ambient CO2, but rather that when the lights are off, our CO2 level in our tank is much higher than would be expected based on the amount in our air. If we truly had good near-100% gas exchange with ambient air for CO2, there would be no pH swing. I tried increasing the aeration with an overpowered air pump and a bunch of air stones to reduce this effect, and it didn’t make much difference in the pH.
Let’s chat about the meter via email. Give it to you during the tour. :)
 

Vincerama2

Supporting Member
Hey V! What is your pH now and have you yet tested co2 in the house? I wouldn't do anything without those numbers.
That said, I have found an air pump outside to be much more effective than just a passive airline. And you can bubble just into the sump rather than the skimmer - most gas exchange happens at the breaking of surface tension, not from diffusion around bubbles. :D

I finally got a new pH probe and fresh calibration (7 and 10) solutions. However, my Pinpoint Monitor display was acting up, so I used an ancient AquaController Jr as the pH monitor. I calibrated it at 7 and 10, but after calibration, dunking the probe back into the 7.0 solution shows 7.02. I guess the Aquacontroller Jr just takes the two readings off the probe and then extrapolates a linear relationsip.
Anyway, using it on the tank I get a pH of 8.28-?8.3X. I assumed it was higher because I use Kalk as my top off and it's unpredictable when the ATO kicks in.

We sat around in the dining room after dinner and I didn't see any dropping down below 8.2.
I'm hoping to get the CO2 monitor soon for interest, but I guess the tank is not desperately low on pH and it's actually pretty good. But I'll check the CO2 monitor to see if it's because my house is a drafty sieve and there is no reason for any extra holes.

I left the skimmer off for an hour and the Aquacontroller Jr shows me a pH of 8.38.


V
 

Matt_Wandell

Supporting Member
Honorary Supporting Member
What's interesting is that a reference from this paper claims:

"coral reef waters during daytime is always accompanied by hyperoxia"

They measure DO between 27% (1.67 mg/L) to 241% (14.19 mg/L) over a full day-night cycle on the coral reef.

Of course, our aquariums are not direct analogs to the ocean. Is the scale of photosynthesis in the ocean enough to create hyperoxic conditions, while in our home aquariums it is not?
Sorry, I missed this, but what paper are you referring to? Those numbers seem insane and not possible in water. It might be measuring oxygen inside tissue or something? Where is the link to the paper?

Edit: Okay, found the original paper. They were basically measuring isolated reef flats at a low spring tide to find those numbers. There it makes sense. Those are numbers the animals tolerate for very brief periods. If you look at a fore reef at 20m where there is always water flow with the larger ocean (and where coral diversity is highest) there is not nearly that much variation in DO.
 
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grizfyrfyter

Supporting Member
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grizfyrfyter

Supporting Member
One of the key points in this article (from reef hobbyist magazine, volume 15) that hasn't been discussed here is that the nitrifying bacteria in your tank are also a major consumer of DO in the water. Preventing or removing decaying organics will help maintain higher levels of DO.
 

PizzaOven

Supporting Member
Sorry, I missed this, but what paper are you referring to? Those numbers seem insane and not possible in water. It might be measuring oxygen inside tissue or something? Where is the link to the paper?

Edit: Okay, found the original paper. They were basically measuring isolated reef flats at a low spring tide to find those numbers. There it makes sense. Those are numbers the animals tolerate for very brief periods. If you look at a fore reef at 20m where there is always water flow with the larger ocean (and where coral diversity is highest) there is not nearly that much variation in DO.
Here is a link to the paper. From my interpretation (and I could be wrong), it does seem like they are referencing DO in the water, not the coral tissue. And to be clear, I do not have a horse in this race! Only looking for thoughtful discussion with the experienced keepers on this forum like yourself.

Appreciate your input.
 
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