Co2 -- Less Oxygen?

[ShinySideUp]

I'm not sure how stupid this question is but hopefully I'm about to find out.

If I have a planted tank and I want to add CO2 to aid plant growth, am I reducing the amount of oxygen available to the fish?

 

[Ps3Steveo]

Increasing it, as with CO2 the plants will photosynthesise faster providing they have the right nutrients. Get a drop checker to make sure you don't have to much CO2 as then the fish will be gasping at the surface.