A quick look at your pond size and plan for two 9" Vertex diffusers @ 6 foot water depth tells me that you are on the right track for at least one turn over in 6 hours. Nice work!
The back pressure on such a system would be in the neighborhood of 5 psi and would require about 2 to 3 CFM. The Hiblow 120LL wants to run just under 3 psi (dotted yellow line) and would only produce 2 CFM at 5 psi.
It would work, but it would be working pretty hard and this would cut into it's life span. And, it would just be meeting the minimum requirement turning the pond over in a 6 to 8 hour night time run. Going deeper with just one of the diffusers could be a pump killer and don;t think about using line with less than 5/8" inner diameter. This system would be on the edge of not working well, especially for nightime only runs.
I tend to over design things, if you have not noticed, and would prefer using a larger more robust pump. But it would mean more money up front...
The Gast 0523 would put out just over 4 CFM at 5 psi and would be too much for just 2 vertex diffusers IMO. It works nicely with three and could run 4 hence more diffusers/lines/valves/etc = more money. I believe the Matala diffusers are rated for higher air flow, but I do not have any experience with them in my pond.
These recommendations are all based on a 850,000 gallon BOW, 6.5 foot average depth, with the diffusers set 6 feet below the surface. Change any of this and the results move around, maybe better, maybe worse. Move the diffusers up and the pump works less hard, but you move less water too which takes longer to turn the pond.
The recent Pond Boss mag has an article in it that talks about DIY pond stuff where it describes, briefly, what happens when too little aeration in put into place. I'll paraphrase (and add a little)...
If the water movement created by the system can not keep up with the constant production of "bad water" from the depths of the pond, then all it's doing is bringing bad water up making the good upper water worse.
As far as the FHM installments, putting more minnows in initially will get you more minnows in a shorter period, BUT it cuts it off at the knees, so to speak, for the long run. Their reproduction rates are exponential, meaning the longer they are allowed to reproduce, the more reproduction you get per unit time. AND, I believe FHM's can start breeding at 3 months of age and re-breed every 5 days after that. You can see how 2 months of time in the pond is not as productive as 5 months.
There is a difference between your 2 month plan and a full year plan. It's not wrong either way, it all depends on what level you are shooting for. How much you want to push the envelope.
