Do live plants serve to provide oxygen

[stanleyc]

I know plants give out oxygen during the day during photosynthesis. But, during respiration, it gives out CO2, so doesn't that cancel out the oxygen that it gave out?

 

[Azhael]

They most certainly contribute to the oxygen concentration in the water. Some fast growing plants like Elodea actually produce bubbles in rather big quantities.
Though it´s true that plants consume oxygen as well as producing it, they consume tiny amounts of it while producing far greater quantities. Given that plants have a very low cost methabolism, this is no surprise.
However, for plants to contribute to the oxygen concentration (and thrive, for that matter), they need to be inside the range of light that is suitable for each species.