A case for adding oxygen

I know that it has been agreed that plants have enough oxygen to get through the nighttime hours, and therefore adding oxygen is unecessary. However, I think I found a way in which adding oxygen does benefit plant growth.

I have a 180 gallon tank, with a completely closed filtration system. I've noticed that my water gets murky a couple of days after a water change (expecially when the tank is viewed the long way through all 6 feet) and tends to stay that way until the next water change. The plants stop pearling, and algae starts to appear. I thought my filter was insufficient, but it seems that's not the case.

I noticed that when the water level in the tank drops due to evaporation, and the filter sucks in some air along with the water, after several hours, the clarity of the water improves dramatically. It is crystal clear even when viewed the long way through 6 feet. The plants also start pearling significantly.

I can only conclude that the added oxygen allows the bacteria in the tank to break down the organic waste in the water more quickly. The resulting clearer water allows more light to reach the plants, and therefore improves their growth. This improvement is most likely limited to closed filtration tanks where there is no water/air mixing other than at the water surface.

Any thoughts?

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On a side note, I was wondering: If I'm bubbling air into the tank during the day as well, will the nitrogen content of the air be absorbed into the water, and if so, can the plants use this from of nitrogen?