Is there any work that has been done to measure the quantity of CO2 produced by plants during the dark period? I am wondering if it is a significant amount of CO2 respired.

I have a 40 gallon sump, in a closed stand below a 120 gallon main tank ("Starting an Amazon System"). The main tank is illuminated during the day, and the sump light is on in the evening. There are growing plants in both tanks, and they are getting nutrients. These questions arose because when I turned off the CO2, the drop checker indicator solution never seemed to turn blue, it stayed green for a long time and at the end of the experiment, 18 hours, it was still blue-green. If I am getting CO2 from the plants in the dark, I'm wondering if it is a significant amount?

I am a newbie, just starting, no critters yet - other than bacteria and fungi. I will make a 4 dkh reference solution, for the drop checker, this weekend to measure the CO2 in the system a bit better. I'll experiment to see what bubble rate it takes to keep the reference solution within the 30 ppm range.

Thanks,
Joseph

Plants respire very very very little. Fish respire a lot more.
Plants do not move nor have many of the respiratory processes fish do.

So 95% of the respiration is from fish, 3% from bacteria etc, maybe 2% from plants.

If the plants start to rot or die fast?
Then bacteria can consume virtually all the O2.

Regards,
Tom Barr

Tom,
Thanks for the quick reply.
J

I believe a lot of us have noticed by observing a drop checker that the CO2 level in the tank doesn't drop a whole lot at night, unless you have considerable surface movement of the water. This is certainly true for my tank. When I switched filters and installed the filter outlet spray bar so it does disturb the surface quite a bit, the drop checker does become blue by morning. Before that, with only small surface movement, it would remain a blue green, tending towards green by morniing.

Yes, That's about right. I'll second that ! The only real surprise I observed was that a properly configured canister filter with spray bar could go head to head with the finest of trickle filters for o2 exchange. I didn't see that coming ? Perhaps I bought into all that marketing hype years ago ? I gotta say in over 30 years time Eheim has never let me down. Planted aquariums are actually very simple so long as you keep it simple !

Sorry to jump in on this thread but I'm a newb trying to learn

So the Co2 levels dont drop much overnight but what about O2 levels?

Without the plants producing O2 is there a big drop in O2 Overnight (and it there anyway to measure this without very expensive kit). Still waiting for my Co2 kit to come before I can start playing with it but this is the only thing I'm really worried about.

Yes, There can be. Depends on the amount of circulation or surface movement and or degassing of ambient Co2. At 5X or more it's generally not a problem so long as the surface of the tank isn't choked with growth, and the spray bar is properly configured. Like wise it also depends on the KH or buffering capacity. Most usually judge by a drop checker or the fishes breathing. Inexpensive test kits are often worth much less than you pay for them ! Still test kits are merely a reference and only as accurate as their confirmed standard. If the Ph rebounds quickly I'd say you have ample o2 providing your KH is at a reasonable level. IE: dkh 4-5 HTH. Prof M

I'm curious as to how Ph rebounding quickly has anything to do with O2, or KH? I'm aware how Ph is effected by CO2, but wasnt aware of any O2 correlation? And KH shouldnt effect CO2 or O2 for the most part either? Could you elaborate on what you mean, this just isnt making any sense to me (or I could just be having a brain fart too)?

You're thinking too hard. It's common sense...not chemistry

They are indirect cause and effect. KH affects PH, and a high buffer would Also cause ph to rebound quickly. (this would be a false positive)

Co2 would lower PH. As Co2 abates the PH rises. In high circulation Co2 abates more quickly. Coincidentally o2 is simply a perk that goes along with the territory.

The only questions remaining are which caused the swing, and how fast ? It's a general observation and unfortunately it's often more accurate than 50% of the test kits on the market... You can do every bit as well flipping any coin.

A stable Co2 content is all well and good, but poor circulation is quite capable of perpetuating that Co2 long after sundown, and once photosynthesis ceases the plants are now competing with livestock for the same o2. Circulation is almost always a Very Good Thing in closed system environments. Co2 very rarely suffocates fish. Anoxia on the other hand is a Mutha ! Grtz, Prof M