I would use 1/2"-5/8" ID Self sink line for aeration line in the pond. Why use Pex when the black poly line is even less expensive for running underground? I'd use 1" line underground. The longer the run, the more resistance and less air flow you will have. That's the reason for going with a larger ID line - less resistance.

You can make your own manifold out of PVC pipe and fittings, using either brass ball valves or plastic ones, although I have noticed that the plastic ones are a lot harder to turn.

Talk to teehjaeh57 on here for the seep repair.