Since the minimum is just that, and there is no margin for error at that level, I'd go with single diffuser disks, and a maximum of 10. 2 psi is 4' water depth. Now, what the people aren't thinking about is how much air line this will take........ Each diffuser disk/airstation has to be at exactly the same depth as others on the same line, or you have to run line from a ball valve to each diffuser disk/airstation.
Case in point. I have 5 2"x12" air stones in Intex swimming pools to hold fish. They are powered by a regenerative blower. Low psi, high volume. If one air stone is 4" deeper than any of the others, the air coming from it is almost nonexistent. I have one air line going to each swimming pool, then around the top edge. The air stones are "T'd" off of that airline.
If it was mine, I would put 4 single disks in one area, making sure that they were the exact same depth, and placed 30 to 40 feet equidistant from each other. That way, if each one opens a hole 30 to 40 foot in diameter, then all the holes will be touching and it will be one big hole. If you think the ones last year only opened up a 20' diameter hole, then place them 20' from each other. I'm not used to running winter diffusers at minimum or close to minimum flow, so I don't know what size hole they will keep open. The wave action from the boil hitting the surface will help keep the ice at bay too. So, maybe 2 sets of 3 diffuser disks each to get more wave action?
Move down a ways (as far as you dare) and do it again.
