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this is a question that has been bugging me for a while, and i believe that i have finally found an answer, in this article:
http://edis.ifas.ufl.edu/FA002
o2 levels in ponds are described. they can get seriously low, during the night, up to dawn, before the sun comes up. this is related to home aquaria, as to how many plants and fish that you have in your tank, and how much water volume that you have, that can dissolve and hold that dissolved o2.
for example, if you have a light fish load with small fish (low o2 levels consumed from the water column at night), with a tank that is planted only say 1/2 of the way up the water column, the amount of o2 consumed will not be that much before the lights come on, when the o2 levels are replenished. but if you have a lot of plants, and many large fish, then the o2 users quickly use the o2 to dangerous levels before the lights come back at dawn.
the answer seems, that if you cannot find a better way to replenish o2 at night, then you must keep the water to fish/plants ratio high, thereby having plenty of o2 dissolved to last the night.
please note, that algae will die off first, before the larger plants will. this may be why the algae all of a sudden disappears in a planted tank, after some time. this is observed by many planted tank owners. maybe the plants are growing fast enough, that they are able to starve the water column enough, that the algae cannot survive through the night. but i wonder what this is doing to the fish? obviously, this last point is highly speculative.
rick
http://edis.ifas.ufl.edu/FA002
o2 levels in ponds are described. they can get seriously low, during the night, up to dawn, before the sun comes up. this is related to home aquaria, as to how many plants and fish that you have in your tank, and how much water volume that you have, that can dissolve and hold that dissolved o2.
for example, if you have a light fish load with small fish (low o2 levels consumed from the water column at night), with a tank that is planted only say 1/2 of the way up the water column, the amount of o2 consumed will not be that much before the lights come on, when the o2 levels are replenished. but if you have a lot of plants, and many large fish, then the o2 users quickly use the o2 to dangerous levels before the lights come back at dawn.
the answer seems, that if you cannot find a better way to replenish o2 at night, then you must keep the water to fish/plants ratio high, thereby having plenty of o2 dissolved to last the night.
please note, that algae will die off first, before the larger plants will. this may be why the algae all of a sudden disappears in a planted tank, after some time. this is observed by many planted tank owners. maybe the plants are growing fast enough, that they are able to starve the water column enough, that the algae cannot survive through the night. but i wonder what this is doing to the fish? obviously, this last point is highly speculative.
rick