O2 may help reduce certain algae, and your fish need it to live.
what reduces Co2 is not the O2 itself, rather how much the
surface water tension is broken by water flow and bubbles popping.
that's why I use a Hydor Ario (with or without LED light) for O2.
it's as cheap as an air pump, tubing, and airstone. it's completely quiet,
and produces fine tiny air bubbles that no airstone could ever achieve.
My Hydor Ario also doubles as my Co2 diffuser since the impeller does
such a fine job chopping up my bubbles into a very fine mist of Co2
that floats horizontally across my tank water.
I prefer mounting it low on your side glass, not the bottom.
the white tube is my Co2, and the black tube is my O2.
both get chopped up by the impeller and released as uber fine mist
that you barely see in this photo as white dots center left and up,
which are finer than some of the best Co2 diffusers I've ever seen.