A windmill could add some aeration to a 7 acre pond, it might not add alot, say enough to maintain oxygen levels required to keep fish healthy but that would depend on the number and type of fish,ie: trout or bass. A deep gravel pit that is 20 feet deep can be aerated with a windmill. At that depth even a suingle airstone being fed by a windmill aeration compressor systemn will provide decent movement which will prevent winter fish kills but if you are trying to effectively aerate a seven acre pond with a single windmill you should have two diffusers placed in the pond at least and we would recommend adding more diffusers depending on the reason for aeration.
We have seen such ponds or gravel pits using multiple windmills, each with two diffuser airstones in the pond to correctly aerate. A 7 acre pond has approximately 305,000 square feet of surface area and a windmill system from Koenders or Outdoor Water Solutions will typically provide at most adequate aeration for one or two acres or 43,000 to 85,000 square feet of surface area.
Doing the calulations you may want to put 3 to 4 windmills on the pond, each with 2 diffusers, into the pond to provide a fully adequate aeration.
Of course that might not be feasible depending on the surroundings and where it might be possible to anchor and place multiple units.
Adding a single windmill aeration system will be better than doing nothing so if you have a decent spot ot anchor and place say a 20 foot or 24 foot windmill then you should go ahead and use this system.
The windmill compresor systems of the Koenders systems or the OWS systems can easily pump air down to the 20 foot depth and you will have no problems with these machines. The American Eagle doesn't have the compressor PSI to develop pressure down to such depths so you should avoid the bellows type of systems in such deep pits. The diaphragm windmills can pump air to up to 45 feet deep but this also depends on the wind viability of the region you place the tower.
I must say that we do not recommend the Koenders windmill systems based on what we have seen but if you have a Koenders Windmill Dealer in your state or county then I suggest you talk to them to ensure your warranty and installation issues can be taken care of by a local windmill installer.
From what we have heard with the feedbck of our clients the Koenders systems are ok. I wouldn't install one on my property but that's my own opinion.
Use a Koenders or OWS windmill in your situation. If your budget can allow for multiple units then you should do so as the CFM pumping rates of any of these windmill heads is going to be at the most 4 CFM and pumping 4 CFM into a 7 acre pond will not leave a huge impact...better than nothing of course.
Details on Windmill Aeration Systems.