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The largest HiBlow that I am aware of is the HP-200 and it is only recommended for up to 2 acre ponds by the manufacturer. It's max depth is 10' and the flow at 10' is about 3.5 CFM (good for 2 to 4 diffusers). Their pump chart suggests that this should only be done "intermittently" at this depth, as well. I don't think HiBlow has a pump for your size pond.
My quick estimate would be that you would need, at a minimum, 5 diffusers in the system. This estimate assumes one complete turn over per day...running 24 hours a day...and a rather round or square pond with an average depth of 7' with all the diffusers at 10 foot deep. I like to size them to do a full turn over in, at least, a half a day, but some do not find this necessary. Cutting the runtime of the pump in half would double the number of diffusers needed (thereabouts).
The pump you would be looking for should be able to pump 1 CFM per diffuser (maybe up to 2 or 3 CFM depending on the diffusers you go with) and be able to do that at 10-12 foot depths. This flow rate will be affected by the line lengths and sizes, so don't just go by the maximum flow rate of the pump. That maximum flow rate is typically without anything hooked to the pump. Once you start adding lines, fittings, diffusers, and water depth...that flow will lessen.
A Gast 1/2 and 3/4 HP pump should be close to the right sizes (Gast 0823 & 1032 models) for the 24 hour runtime. I say "close" because it all depends on the size and lengths of line you choose to use. The longer and/or smaller the lines...the more pump you need. There are other types of pumps to chose from...I just happen to be most familiar with the Gast brand. I would consider using 10 diffusers and 2 pumps. This would allow you to only run at night (half day) and would allow for one system to go down and still turn the pond (in a 24 hour run). This would be a bit of and overkill if the budget is tight, but could be very flexible if the budget was there.
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