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Re: Trouble with CO2

John wrote:
"Seriously, should I be surprised that the fish couldn't breath at a KH of 3
and a pH of 7.4 up to 8? In the past, a KH of 1 and a pH anywhere between
6.4 and 7.4 didn't seem to affect the fish. Also, sometimes I get the
impression that the recommendations I read on this site (and elsewhere)
refer to plants but are not good for fish, particularly discus. Is this
balancing act between keeping discus and growing plants more difficult than
with other fish?"

There is nothing incompatible between proper planted tank maintenance and
the provision of a safe, healthy home for Discus. Adding CO2 to an aquarium
will NOT displace the Oxygen the fish need for respiration (within reason,
of course) and if you checked most well run planted tanks with injected CO2
you would find Oxygen levels at or near saturation values. On the other
hand, there ARE many traditional practices common in the maintenance of fish
only tanks which are of doubtful use or downright counter productive when
applied to a planted tank.

On the KRIB, at http://www.thekrib.com/Plants/CO2/co2-level.html , there is
the following quote:
"Neon tetras, Angels, and Discus, for instance,
      show absolutely no signs of discomfort even at 100 mg of CO2 per

IF the parameters you report are accurate, and IF you don't have some other
buffer system in your water skewing the results, your CO2 levels are low and
would pose no danger whatsoever to fish. According to the pH-KH-CO2 chart,
at a KH level of 3 and a pH of 7.4, the CO2 concentration is approximately 4
ppm, which would pose no danger to your Discus at at, but not do that much
for your plants either. The optimum level of CO2 in a planted tank is in the
range of 15-20 ppm. Increasing your KH to 4 and injecting sufficient CO2 to
drop your pH to 6.8 would result in a CO2 concentration of approximately 19
ppm, still quite safe for your fish and very beneficial to your plants.

The use of the chart also requires that you have accurate readings for both
KH and pH - not all hobbyist grade test kits are up to snuff for this
purpose. Are you using any specialized "Discus" additives in your water?

From your post, I suspect that perhaps you might be coming at this from the
wrong end:
"A few weeks ago I asked members of this group for advice on raising the KH
and using CO2 injection to adjust the pH of my freshwater tank (KH =1)."

It sounds like you consider the injected CO2 as a means of lowering the pH
of your water. It does have that effect, but it really isn't the purpose for
which you inject CO2 in the first place. The CO2 is meant as a source of
Carbon for the plants, not as a method of pH depression/control.

A lot of hobbyists cling to the belief that fish NEED a particular pH level
for their health and happiness. While it might be needed for breeding
purposes for some species, a particular pH level isn't all that necessary
for the normal matinenance of very many fish - I have some very healthy
Discus in a tank with a pH of 7.8 water. Maybe my discus just haven't read
the bits about them requiring a pH below 7.0. I'll be sure to keep those
articles away from the tank, so the fish don''t get the impression that I'm
being cruel to them.....;-).

Does your tank have adequate circulation? Your problem might be due more to
inadequate circulation and too many fish than it is to the injected CO2.

Keeping a well planted aquarium with happy, healthy fish isn't all that

James Purchase