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Low oxygen vs. high CO2

A month or two ago, there was some discussion of low oxygen and/or
high CO2 killing SAEs after plant pruning.  I now have another direct
experience that might give a little more insight.

As you may recall, I had reported that after performing a major
pruning in a tank with minimal surface agitation, I woke the next
morning to find 3 (of 3) dead SAEs and 1 (of 4) dead clown loach.
Other fish were suffering, but all recovered quickly when aeration
was added.  I did not connect the pruning and the deaths until 
a while later when I pruned again and found my 3 replacement SAEs
suffering severely while other fish were gasping at the surface.
Again, aeration provided quick relief.

I theorized that this was due to a low oxygen condition caused by
increased plant metabolism when repairing the damage caused by 
pruning.  I do not believe that this was high CO2, since it occurred
at night while the plants would not be absorbing CO2 anyway.  

Well, the other night I inadvertently performed an "experiment"
which lent more credence to my theory.  I had adjusted my CO2 flow
to slightly increase it.  When I woke in the morning, it seems that
the valve had drifted somewhat higher after my adjustment, and the
CO2 flow was quite high.  All fish (except a betta) were showing
signs of distress.  But this time, the SAEs did not seem to be the
hardest hit.  In particular, a favorite old pictus cat (who had
survived all my previous "experiments" with apparently minimal
effect) was quite dead, and I was extremely concerned for a
couple of kuhli loaches that looked very bad.  SAEs were sitting
in plants near the surface with gills pumping rapidly, but swam
around a little when I disturbed then by frantically setting up

I cut the CO2 flow and added aeration, and everything recovered,
except the pictus cat, of course.  I noticed that the recovery 
was distinctly slower than it had been from the low oxygen
occurrences.  The behavior of the fish in the two cases was also 
quite different.  While the low oxygen caused fish to gasp at the
surface, the high CO2 made the fish act lethargic while pumping
their gills rapidly.

I conclude from this that the SAEs are more sensitive to low
oxygen conditions, while other fish (such as the pictus cat or
George's rainbowfish) are more sensitive to high CO2 conditions.

BTW, I now aerate my tank at night if I have pruned during the
day.  And I will, from now on, only adjust my CO2 flow in the
morning and check it again at the end of the day to make sure
there hasn't been too much drift.  I hate killing fish,
especially through incompetence.

David Ozenne
Berkeley, CA