Your pond water with 16"-18" of visibility / transparency means light is only penetrating and algae making dissolved oxygen to around 3ft - 3.5ft. Aeration would be beneficial in this pond depending on what is causing the transparency; be it mostly silt/detritus and/or plankton. .
There's roughly 4,480,451 gallons of water in the pond. Typically with a bottom aeration system you want to bring a minimum of 1.2x the water volume to the surface every 24 hours to get it in contact with the air to transfer O2 to the water. (cody i.e. one turnover per day).
I don't think that a fountain @ 210 gpm by itself will be sufficient.
210gpm is 12600gph. 4,480,400g/12600g/h = 355hours/24h= 14.8days running the fountain 24/7 to get one pond turnover. However there could be an oxygenated refuge in the area around the fountain - depending. IMO do not buy that fountain for any measurable aeration benefit for the outer reaches of the one acre pond. This is especially true if you do not run it 24/7. Typically fountains are only good for mixing and aerating the same water over and over in the near-by vicinity of the fountain. They basically mix the same water over and over and do not spread the water currents outward away from the fountain unit. Water is sprayed upward until mean Ol'Mr. Gravity has the water fall straight back down around the fountain. Mostly a energy force/push downward - little if any outward water spread. Watch how it works and the resulting water movement. IMO if you pull deep water to the surface, this mixed blended water will stay in the vicinity of the fountain. Deep water is significantly colder more dense and will tend to sink back downward below the fountain. Thermal and DO testing will prove this. However the fountain will be pretty and nice to look at. Pretty things including women cost money. Often lots of money for the visual and personal pleasure.
My thermocline, or I assume it’s thermocline, is around 6ft down, that’s while swimming the water gets a lot colder at that depth. The clarity in the summer is about 14”
I think your estimate is very close to the actual summer thermocline depth close to 6ft. That depth in my testing is very often the natural summer wind mixing depth of small ponds.
