I dun think it matters whether you placed your diffuser near the fan or not. It is when the small bubbles rising to the surface dissolved the CO2 into the water. Once it reaches the surface, it will escape into the air. I put my diffuser near the outflow of my filter so as to "push" some of the CO2 bubbles to circulate the tank.
It is also true that surface movement increase the rate of CO2 escaping from the surface. But I reckon some surface movement is good so that there are enough O2 for the fishes and especially shrimps.
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