Having a lot of CO2 does NOT mean that you won't have any O2! Quite the opposite in fact.
When CO2 is high and plants are really happy they'll produce oxygen bubbles (this phenomenon is called perling). This only happens when the O2 saturation of the water is past 100% and it can't dissolve any more oxygen.
Keeping your plants happy by providing enough CO2 is the best way to introduce O2 into your water.
However, if you ever overdo the CO2 it can harm your fish. The way to find the threshhold is to keep turning it up until you see the fish hanging out near the top, then turn it down slightly. Be patient and adjust slowly. Do NOT drastically turn knobs and then go to sleep for the night leaving your CO2 running . Most people turn it off the CO2 at night. If you do OD on CO2, open the lids to the tank, do a big waterchange, and run an airstone. Many times they recover 100%.
this is an excellent way to oxygenate your tank
AND at the same time diffuse Co2. Hydor Ario 2 aerator
get the Ario-2 which is big enough for a 60.
to use as a Co2 diffuser, simply place that
second hose in front under the foam filter.
the bubbles come out FINER than
even the best diffuser or air-stone.
the only thing that does a better
job is a reactor which takes up
more space, and can be noisy.