I'm renovating my pond and just used a temporary syphon to drain it. Does anyone think you could possibly make a syphon that had a bypass that just circled back into the pond creating an aerator?? (I'll accept laughs as replies!)

Does your pond have a continuing inflow? Like, constant spring fed or continuous running creek feed, or natural water flow very near by.

May be able to suck air in from the surface and later letting it out when flow through siphon is sufficient using venturi effect. Never tried it though. This would reduce outflow capacity though. If the inlet for the air is set just maybe a foot above full pool, then this would be minimal but would not aerate during high water because air inlet would be under water. might also need a check valve on air outlet to keep water from draining pond during low water.

I think you exit has to be several feet below your input. Don't think it would work....

Siphon flow depends on a pressure gradient. Specifically, the pressure at the syphon outlet must be less than the equivalent elevation in the reservoir. When the end of the syphon is connected to the reservoir the pressures are the same and so there will be no flow. The concept you imagined is but one of many imagined "perpetual motion " or "free energy" machines. They seem plausible until one considers the physical laws at play.