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Botanist question

About 6 weeks ago I increased the light over my tank to 3watts/gal.  Over
the course of time, the plant mass has grown dramatically.  I also noticed
that CO2 has become the growth limiting factor because KH has gone down to

As a stand-by measure while I research and pursue the parts etc. for a CO2
setup, I thought that perhaps, if I decrease the duration of light, my pH
would not bounce as dramatically, and maybe the plants would slow down a

Well,  this morning I noticed that my fish were not exactly puffing, but not
exactly looking like they were not exerting effort to move water over the
gills.  So I measured the O2 level, which was less than 8mg/l and above
5mg/l according to the Tetra test.  (This in contrast to last week-end when
hours after a water change, the plants were fizzing and the O2 level was
super-saturated 14mg/l+).

I really hate to see my fish looking stressed, so I adjusted the spray bar
in order to create some significant surface movement but not turbulence.
And flu or no flu, I am going to have to hoist my petard and check out the
welding and valve places today.  (I just looked up "petard" in the
dictionary, and it means "little firecracker".  LOL.  Never checked before.)

Anyway, to get to my question:  My understanding is that plants fix CO2
during the day via photosynthesis, utilize O2 and produce CO2 at night .
From my "lay" observation, of for example pole beans, plants grow during the
night using up the energy they trapped during the day.

So, in the aquarium during the night, both plants and animals give off CO2.
By tipping the balance in my tank, i.e. reducing the light duration from 12
to 9 hours per day, it would appear that the amount of O2 being used up at
night was great enough to cause sufficient depletion that the animals became
uncomfortable.  The only parameter/variable was the duration of the
light/photo period.   Assuming that neither the animal O2 load nor the
surface water movement  was changed, then the only cause of the low O2
situation were the plants.  I think that this resulted from the fact that
with the photoperiod reduction,  the probable O2 saturation during the day
was not attained.  However, based on probable optimum (under the
circumstances) CO2 fixation during the day, the plants were still capable of
optimal O2 utilization during the night.  So, for every milligram of CO2
fixed, how many milligrams of O2 are used up by the plants?  In other words,
what is the ratio of CO2 fixation during the day to O2 uptake + CO2
production of the plants at night?  I am directing this mainly to our
friendly botany professor in Mississippi.  It has become clear that it is
this difference that has tipped things adversely (but only temporarily) in
my tank.  The question may appear esoteric, but puffing fish are sadly an
"in your face" phenemenon.

It would seem that given the circumstances, my fish have me backed into a
corner.  So, off to the Yellow Pages I go.

G. Kadar