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[APD] RE: CO off at night

With all this discussion of CO2 day and night, I thought I would do a
little checking on things so I used my 60 gal. cube aquarium for the
following observations.

The aquarium is set up as a slow-grow tank housing Discus, Cardinals, etc.
that gets some filtered sunlight on two sides and has a 2x20W shop light
sitting on top. Its covered except for a 3" gap. Filtration is a Fluval 404
canister with nothing but the foam pads and ceramic rings that come with it
and I run a sponge prefilter on the intake. CO2 is injected into the intake
by a pressure system. I just stop the filter when I service the tank and do
a rough "bubble count" at this time. Actually, I just look to see if the
bubbles are occurring at the usual rate that I have figured out to keep the
pH at a certain level and therefore the CO2 concentration is adequate. I
keep the KH around 60 mg/L (~3 deg.) and the pH measures 6.9 in the morning
so I think I'm getting enough by the chart figures although some might
think its too low. The afternoon readings are slightly higher. This tank
has been set up for about 3 months now.

I have been noticing a slight drop in pH as the plants fill in and I was
assuming this was a natural occurrence from an accumulation of fish waste
and other natural processes as the tank ages even though it gets 50%/week
water change. Yesterday I was looking at the tanks early in the morning, in
the predawn light, and I did notice all the fish breathing heavily. I did
both a pH test and a CO2 test.

The pH was, as expected, on the downward trend that it has been on for the
past few weeks. It measured at 6.7. The CO2 measured approximately 54 mg/l.
Now,  I remember a post a while back about the reagent used in the CO2 test
becoming contaminated by atmospheric CO2 over time and therefore making the
test results invalid. Can I trust the test? I don't know. Its a good
quality test kit but the reagent has been opened and used for several
months now. Both the KH and GH measured at 60 mg/L (~3 deg.) so the CO2
content, by the chart, should be around 20 mg/L if nothing else is causing
interference. Are there any other substances in the water causing
interference with the table calculations? I don't know. I don't think so
but I can't be absolutely sure.  Whatever this discrepancy is really
doesn't matter. The fact is the fish are definitely in need of some oxygen
first thing in the morning. After daybreak, the fish calm down and when I
usually see them in the afternoon they appear quite normal.

After reading the APD posts and looking at how the tank has matured over
time I think the problem is the fact that I have let the Pistia grow on the
surface to the point that it is covering the entire surface and blocking
the gas exchange with the atmosphere. I'm going to remove most of it and
see what happens. Hopefully this will restore the normal off-gassing that
keeps the system stable.  


--- Eric.

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