# Plants absorb O2 at night?



## sharkerbaby (Jul 25, 2004)

This has been posted over and over on another board I'm on and wanted to check with you all. It's been used to explain unexplained fish deaths at night - low O2 in water column because the plants absorbed it. This just doesn't sound right to me. I don't think terrestrial plants do this, why would aquatic plants?

thanks for any insight,


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## DonaldmBoyer (Aug 18, 2005)

Plants do require oxygen for respiration at night, or when the lights go off. This is true of every single plant, both aquatic and terrestrial. However, the amount of O2 needed for plant respiration is low, and it is likely that the person does not have adequet filtration. Filters dissolve a high amount of O2 into the water, and unless the person's filters go off during the "dark period," they should supply plenty of O2 into the aquarium.

Another possibility is that said person may have too many floating plants in a very, very heavily planted tank which could deplete O2 at nighttime, and also lead to a HUGE drop in pH too, as the plants produce CO2 during respiration. So, he could be experiencing fish loss due to the resulting lack of O2 and severe drop in pH during his "dark period." However, this is uncommon, and he should really look into adding more filtration or increase his water movement with a powerhead.

I learned this in my Plant Biology and Plant Physiology classes in college, but as a quick reference (so I don't have to dig through my old college texts), I reference p.74 in Peter Hiscock's Encyclopedia of Aquarium Plants


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## Left C (Jun 14, 2005)

Plants use oxygen 24/7 for cellular respiration just like most other living things. Even during photosynthesis they are consuming oxygen.

See page 6 of Diana Walstad's book: Ecology of the Planted Aquarium.

*"*Actually, the air probably provides more oxygen consistently to fish than plant photosynthesis. And while it is true that plants also consume oxygen (plants 'breathe' just as humans do), healthy plants give off far more oxygen via photosynthesis than they consume by respiration. Even when plants are not photosynthesizing, such as night, they probably remove much less oxygen than one would expect. This is because they prefer oxygen stored in their tissues rather than take up oxygen from the water.

During photosynthesis, oxygen accumulates rapidly within the plant lacunae, which are huge gas storage areas making up about 70% of the plant's interior. This internal oxygen is used for the plant's respiration both day and night.*"*


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## sharkerbaby (Jul 25, 2004)

Well, isn't that fascinating. Thanks I learned something today


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## fattyratrat (Jun 19, 2007)

Most professional plant people keep air stones/sticks on timers opposite to their lights... becasue CO2 dissolves in and out of the water so quickly, the slightest bit of extra bubbles(AKA surface area) will help get exess CO2 out and prevent extreme PH swings.


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## imported_jewels (Oct 2, 2004)

you can buy an oxygen test, and use this to test your tank late at night. This will help you to get a better idea if this is an issue for your tank.

J


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## sharkerbaby (Jul 25, 2004)

Thanks for the reply. Thankfully, it's not my tank that's having the problem. Nothing more has been written about it so I gues whatever it was has been resolved.

thanks again


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