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Re:CO2 and O2

> Okay, picture this: a tank with power filter, NO plants, but somewhat
> overstocked with fish, and completely cycled.

You will be taking O2 in from the air, and giving CO2 off to the air in this

> SUDDENLY, you're adding CO2,
> the line going *directly* into the intake of the power filter.

Your simply giving off more CO2 but it's a slow process. Add aeration etc
and this speed of this process increases.

> Now, would 
> this affect the biological filtration to the point where you'd see an
> ammonia spike? Remember - still no plants. The "consumption" of the ammonia
> is still only the Nitrosomonas and Nitrobacter.

You'd have to add more than 35ppm. Maybe as high as 100ppm. Even then......
 Since the CO2 is being
> efficiently diffused into the water by the impellor chopping up the bubbles
> *right before* going through the bio medium (not to mention the localized
> acidity), I see this killing off a good portion of the colonies in the
> filter.

Well not so much killing them as reducing the pH down way below their
optimum level to do their work( optimum is 7.0-7.2 or something close I
recall). At a pH of 5-6 they do not do much work and go on strike. A KH or
3-5 is good since your optimum pH is then 6.4 to 7.0. My KH is higher and my
pH is in the 7.0 -7.2 range.
I _never_ add CO2 into my filter. Some folks have good luck with it but I
don't do it.

> I'd also imagine that in time, re-colonization would occur in some
> spot that were chemically "less hostile" to their existence. Or am I wrong
> again, and this would not affect the bacteria in the filter in any way?

It will and does. But this new spot will not be as condusive as the
filter(nice and dark, lots of water flow, high surface area etc)

> Don't ask about the logic of the scenario and the lack thereof. I just want
> to know if a "high" concentration of CO2 would indeed adversely affect
> aerobic bacteria, where they were densely populated.

Yes but not due to pushing the O2 out but rather the low pH's associated
with the CO2 acid additions. There could be other things happening but this
is the main short quick answer I can think up.

At higher KH values the effect of the pH will be small while folks trying to
ride the lower edge of the KH values(3 or less) are ***much more** prone to
experiencing a pH/bacterial issue since their associated pH values will be
much lower than say my own(KH 8). Some folks have KH's less than 2 or 1 and
the pH when using CO2 gets under 6 often times if they are trying to get
20-30ppm of CO2. This cannot be good for filter bacteria. I would have to
kill my fish with CO2 toxicity(well over 100ppm of CO2) in order to harm my
bacteria with the pH. I think many bacteria can handle the CO2 as long as
there is O2 available, it's the pH that bugs them.

Tom Barr

> Thanks for any responses.
> -Naomi