Fountain aerators draw or suck water from basically the intake depth. So if the fountain intake is 5 ft deep then the fountain by itself will not cause mixing much more than 5 ft deep. Note many fountains have intake depths of 3ft. Wind will help circulate the surface water as deep as 8 ft if you get big waves 2ft to 4 ft high. If you have a fountain with a 3 ft deep intake and oxygen is present at 6 ft deep, the oxygen is being circulated to 6 ft deep by wind, convection, or boat motor mixing.
A fountain sucks up a lot of the SAME water and sprays it into the air over and over. Thus you can get a real high amount of dissolved oxygen in the area around the fountain. This area would be a high oxygen refuge area if you have SUMMER turnover or inversion and the top water loses its oxygen. Trouble is will the majority of the fish be "smart" enough to find this oxygenated water. The other factor that comes into play is the size of the pond or lake. The bigger the water body the more fish there will be and what chance is there that the majority of fish will find this high oxygen area fast enough before they suffocate.
"They" say a fountain is a good emergency aerator due to the way it operates and how it adds large amounts of oxygen right around the fountain. Are the fish smart enough to go to this newnoisy area if a motorized spraying fountain is added during a fish kill? Normally wildlife is pretty wary of anything new added to their environment and they initially shy away from it.
A bottom aerator - diffuser elimiates all the above problems because it continually or daily recirculates quite a large portion of the WHOLE area around the diffuser/s; BOTTOM to TOP. Thus the pond bottom water does not build up a large oxygen loss and a sumer turnover or inversion rarely happens that results in a major fish kill.
The other major benefit to a bottom aerator is it keeps oxygen at the bottom and across the bottom muds to keep tremendous numbers of pond organisms alive at the bottom. Pond organisms on the bottom help decompose the organic material that accumulates / settles on the bottoms.
When fountains are used in ponds where the bottoms are deep (greater than 8ft), all deep bottom areas lose the oxygen during summer and all bottom dwelling insects, invertebrates, mollusks and general life forms leave or die. Fish also leave this zone. Why would you want this to happen? Why cut off the oxygen to a part of your body? What will happen to the oxygen deprived part then? Also when oxygen is LOST on the deeper bottom muds the decompostion rate or decay rate drops 20 - 30 times; primarily because all the life decomposing processing forms have died or migrated out.
In every pond deeper than 8ft where I have tested the oxygen when fountains were used during summer,, the oxygen is gone or down to 0 in June/July. Sometimes it is gone as soon as end of May. "Fountain salesmen" ignore, do not mention, or are not aware of this very important point. If they say a fountain mixes or draws water from the deep (8ft+) zone they are not being truthful or/ and have never tested the mid-summer oxygen levels in the deep water with a fountain as a mixer.
In summary, in deeper ponds, fountains do not circulate bottom water and bottom diffusers do.
Do you want oxygen on the deeper bottom areas or not? It's your choice and it's your pond.
