I know it has been said before but I think it sometimes get lost with all the emphasis on preserving CO2, however, in my semi-lowtech* I've noticed the plants were in decline. Had been for some time but, given that I'm planning to do a new tank, I was inclined to leave it to see what happened over time. Time passed... and the beginnings of some surface scum made me decide to drop the lily pipe (heffer of a ugly plastic thing) and direct the flow to increase surface movement and increase oxygen exchange. Anywaaaaay... The plants have picked up. The surface scum was minor so I'm reasonably satisfied that the "extra" light is not making the difference. That leaves me to conclude the oxygen was depleted. So, I'm losing CO2, gaining oxygen, and improved plant growth. All un-scientific... * Removed CO2 and 3rd tube from a high-tech.