Omaha, just to give you an idea, this is what a 4/0 wire looks like: (its Aluminum)
A wee bit bigger than 12 AWG, and there would be 3, plus a smaller ground wire. Didn't have any 350MCM Aluminum laying around, but it's larger.
This would be for 60 amps at 240v running 750 feet (copper). You won't need that tho. It was just an example. If it were me and I was going thru the expense, I would at least run 60 for having greater future options.
At least consider 30 amps to the pond. That would be 3/0 Aluminum for full load, but you can usually tweak that down based on what you might be running at the same time to 2/0. Also don't know how picky your inspectors are.
Based on the info TJ posted and looking at some Grundfos pump curves. Probably could get handily by with a 3/4 hp pump. I would probably bump it up to a 1.5hp 3 phase 5 stage pump, and toss a VFD on it. Tweak it back to 25gpm and it will eat substantially less watts than the 3/4hp single phase pump. Plus, you have the option of cranking it up!
Yeah, it can get complicated!
Figure a couple grand for aluminum direct bury cable on the minimum side of things to make the motor spin.
