Very good observation and you are correct. The only problem with using submergible pumps is that they heat the water a bit as well. Heating the water is not a good idea, cool water can hold more oxygen then warm water will.

The more air bubbles you have the more agitation you can produce with the bubbles and thus the more oxygen you can make.

There are many growing styles that use the capillary action from the air stones to provide water the roots. They are a viable way to grow hydroponics for many starters.

So you must consider balancing the air vs. the heat. You have observed I assume; it's the falling water that reproduces the capillary action and oxygenates the water. That can make noise as well (the falling water fall) so if your goal is to quit the noise from an air pump then maybe another pump may not be the best way to go. They do have many quit air pumps but you also make a great point as well.

It depends on your growing style and your choice of methods you use.
If you switch to the outside pumps to save the heat then you have the sound of the water falling back into the rez to deal with.
All of the above methods can and do provide plenty of oxygen for the plants, One thing we have over fish thanks is that for a proper hydroponic grow you change out the water and nutrients every 7-10 days anyway so what you are doing with the air stones is to not only water the plants if that is how you are growing it also does oxygenate the water.
So give some more thought before you toss those air stones in the trash.
Zandor Reviewed by Zandor on . Air Pump vs Water Pump I believe that using diaphram air pumps to oxygenate res water may not be the best way to go. This opinion is based on many years of experience in setting up and maintaining reef aquariums and researching techical information relative to the health of delicate corals. As I understand the air/water exchange principles, the actual gas exchange takes place at the surface where the air and water come in contact. The agitation of the water causes an incease in the volume of oxygen in the water. Rating: 5