Hi, all,

Several planted aquaria club members got into a debate about CO2 and
surface agitation a couple of nights ago. Below is the consensus we came
to. I think we got our facts straight in the end, but there are a couple
of extreme cases we wanted to confirm.

In all cases, assume a planted tank with both fish and a cannister
filter.


NO CO2 INJECTION:
Surface agitation is GOOD: without it, plants will extract the CO2,
driving pH up, particularly in tall tanks. CO2 levels in non-injected
tanks don't go above about 3ppm regardless, so surface agitation keeps
the level at or near that point. Fish don't exhale enough CO2 for the
plants, so it is much more productive to encourage CO2 diffusion in from
the atmosphere than to try to retain the fish output.


TANKS WITH CO2 INJECTION:
Surface agitation is BAD, as it allows precious CO2 to outgas. However,
fish need O2, so a balance must be struck. Plants produce O2, but only
during the light period, and they consume some during the dark period.
Larger fish will typically be the first to start gulping air at the
surface when not enough O2 is present. Unfortunately, pl*cos and some
other fish can suffer badly from inadequate O2, so experiment carefully.



OPTIONS FOR INJECTED TANKS:
Maintain enough surface agitation such that fish never gasp at surface.
Pay close attention to fish in the morning before lights come on, as
this is the point of lowest O2 concentration in the water.

Consider adding an air pump and air stone which comes on when the lights
go off. This will increase circulation when it is needed most, and when
CO2 is needed least.

If you have PRESSURIZED CO2, a little more surface agitation is not
quite as critical as with DIY, as bottled CO2 is relatively inexpensive.
My own experience was that increasing agitation meant I needed to
increase my bubble rate only incrementally to maintain the same pH,
raising annual CO2 costs from about $12 to about $15. The increased
agitation allowed sufficient O2 that the fish never did their gasping
act.


INJECTED TANKS WITH TIGHT COVERS
In this case, outgassing CO2 can accumulate under the cover, displacing
atmospheric O2, which will dramatically lower O2 levels in the water.
The only solutions here are to loosen the cover, or to add another O2
source, typically an air pump and air stone.


VARIABLE CIRCULATION RATES OF CANNISTER FILTERS:
As the filter material clogs, cannister filter circulation rates tend to
go down, decreasing circulation and surface agitation. You may have
enough agitation to keep O2 levels high right after you clean the
filter, then be severely O2 deficient as the filter clogs, especially if
your bio load is high or your filter is small for the tank.


WHEW! Did I get it all? Thanks in advance for any corrections/updates.