If I don’t do surface, and do bottom instead, how many would I need?
A 3 ac pond 15ft deep in KY should rarely ever need to be winter aerated. I do not winter aerate my 15ft deep pond in Northern Ohio. I am prepared to aerate it but the need very rarely happens.
esshup has this concept correct. Also the shape of the pond with all having the same area makes a difference for the number of aerators. A round pond is easiest to aerate. Round ponds have the least amount of shoreline and tend to need less mechanical mixing. . Aerator influence is circular or bowl shaped. for both fountain and the most common bottom based disk or tube aerators. The further you deviate from the basic bowl shape the more mixing that is needed for the out lying areas of the main bowl basin.
Depending on exposure to wind and height of the wind created waves,,, most smaller ponds usually do not wind-wave mix much deeper than 7 feet. Fountains as normally sold 'out of the box', mix just that upper warmer 6-7ft layer. Deeper colder water is more dense and has less tendency or more resistance to mix with the warmer upper layer. This is all due to density or weight of water. Large ponds with good wind exposure that create white caps and deeper wind / water mixing can periodially mix as deep as 9 to 10 or maybe 12 ft; exceptions can be deeper. Depth profile temperature testing determines the depth of the natural water column mixing of EACH different pond.