To help clear up some confusion, what gets used up is "Dissolved Oxygen", which is oxygen that's in the water, that fish and plants need to survive. Fish and plants aren't separating the hydrogen and the oxygen in the water molecules, they're using the dissolved oxygen suspended in the water.

Some ways to aerate water are fountains, bubblers, or in the case of flowing water you just put a bunch of rocks in the water to create rapids, also called riffles.