If you have a beautiful waterfall cascading into your pond churning up the water and creating lovely sound and water movement it would be understandable that you would think that you are giving your pond adequate aeration. Unfortunately while it does assist with gas exchange and infuse oxygen into the water this will not always be enough. This is especially true if you only run your waterfall pump for a short time.
When you have gallons of water coming over your waterfall you are increasing the surface that is exposed to the air and harmful gases are released. However you also want the water from the bottom of your pond to circulate and a waterfall will not always help with this. The pond bottom has less oxygen than the upper layers of the pond especially if you have stone or gravel in the bottom of your pond. Anaerobic bacteria work in the absence of oxygen and cause a septic condition in the bottom of the pond. When you expose the bottom of the pond to oxygen rich water you will have aerobic (uses oxygen) bacteria breaking down the organic matter. This will reduce foul odors in the pond, which are caused by anaerobic bacteria.
If you have signs that your pond is lacking in aeration then your waterfall probably isn’t cutting it for aeration purposes so you may want to add an air pump to your pond. This will increased circulation and gas exchange. Signs that your pond may need an air pump would be your fish gulping for air at the surface, extensive algae blooms and mosquito hatches. If you have one or more of these it is probably time to increase the aeration and circulation in your pond and a very effective way to do this is by adding an air pump which can run relatively inexpensively constantly and ensure the health of your pond.