HI, I'm a little unclear about the proper use of a power head. I get the flow part. But I read some posts that claim the benefit of increased oxygenation (and increased CO2 also) from the increased surface agitation. But isn't surface agitation bad, for offgassing CO2? I am planning a low-tech non-CO2 (maybe DIY) 75 gal. What is the best use of a powerhead in that situation? Thanks for any advice!