I'm no expert on aeration, but can say that in my 3 acre pond I'm using three double 9" diffusers so I would think one 9" for a half acre would suffice.
You might reconsider on the catch and release policy because likely some judicious removal of some numbers of one type fish often improves the over all fishery. A pond can only hold so much biomass and often one type will gain advantage and limit other desirable species. Careful removal of some select fish can improve what is left. Something to think about.
