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Re: O2 levels

Daniel Larssen wrote:

  > I have readings of 2 mg/L O2 in the
  > mornings and 6 mg/L O2 in the
  > evening. Kesselman recommends a
  > value of 5 mg/L - even at night.

Daniel, my reading suggests that oxygen stress sets in for many
warm-water fish at about 4.5 mg/l.  If your tank reaches 2 mg/l then you
should have some dead fish.  How are you measuring O2 levels?

I once had a rather expensive oxygen test kit.  It only provided 8
tests, so I didn't experiment much. Each test was a "winkler" test
contained under vaccuum in a single sealed glass ampule.  The tester
would take a sample of the water to be tested in a special container
then place the narrow neck of an ampule into the sample container and
press down.  The sample container was made to snap of the end off the
ampule.  The ampule was under vaccuum, so once the end was snapped off
the ampule would suck up a pre-determined amount of the sample.  The O2
level was determined by shades of blue.

I found that my tank was at about equilibrium levels (about 8 ppm) with
atmospheric oxygen when the lights came on in the morning.  In the
evening with a few plants bubbling the oxygen level pegged the test
scale at 10+ ppm O2.  I also used the kit to test my tap water, which
came out with about 6 ppm and my well water, which came out with 0
oxygen, both of which made sense.

The tank I tested was modestly lit, moderately stocked with fish and
wasn't CO2 supplimented.  The tank was well circulated but not aerated.

A healthy planted aquarium without very large livestock populations
and heavy feeding should go over atmospheric oxygen levels in the
evening.  That means something like 8-9 ppm of O2.  How low it drops at
night should vary with stocking levels, circulation and aeration.

Roger Miller