You can have the powerhead anywhere in the tank because once the water is circulating, any CO2 will also be circulating. However, most people put the powerhead near the CO2 bubbler, but it doesn't make a lot of difference.
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Out of curiosity, do you have lots of plants in the tank and are they fast growing plants?
Do you add lots of plant fertiliser?
If you don't have lots of fast growing plants and aren't using plant fertiliser, there is no point adding carbon dioxide (CO2) to the tank.