Here is a formula to use

Surface Acreage Area:
A) Multiply the length in feet of the lake, by the width in feet of the lake. Now you can calculate the surface square foot area of the pond or lake. So your Area = L x W
B) Divide the surface area (in square feet) by to get the number of surface acres. Surface Area = Number of Acres. So 300 ft in width X 520 length= 156,000. An acre is 43,560 sqft, so take 156,000 and dived it by 43,560= 3.58 acres. So a 100x200 pond is 20,000sqft and 20000 divided by 43,560 is .45 acres so a little under half an acre. I would think like in septic you are really after D.O dissolved oxygen.

Calculate the number of H.P. pumps needed:
Number of aerators in H..P. = Number of acres (rounded upward)
Example: The lake is 220 ft. Wide x 370 ft. long (220' x 370' = 81,400 sq. ft = 1.87 acres (rounded up to 2 acres).
A 43,560 Sq. Ft. Lake requires either: (2) 1 - 1/2 H.P. aerators for normal conditions or (2) 2 H.P. aerators for warm climates or lakes with a history of high algae problems.

I would think like in septic you are really after D.O dissolved oxygen. If you have a ton of fish, they will need more D.O. to survive and thrive. I had a guy wanted to aerate a 33-acre pond a small lake if you ask me. That's hard to do as if you aerate one area the D.O will raise but more so on one side of the water body that size makes it hard to penetrate the whole thing without spending some severe money. So its not always this way I cant say hey this R7 Gast at 33 hp is going to work and cost 47,000 dollars.Larg water areas have more needs and complicate things.

Hope this insight helps