I have a 35G pond liner made of heavy duty plastic that I use in my duck pen to keep water for the ducks to swim and drink. Unfortunately during the summer heat the water goes sour after about 24 hours. So I have to drain the pond, rinse it out and refill it every other day (not an easy task). The liner does not have a drain; it's like a big rubbermaid "tub".

I don't plan to keep the ducks much longer, so I'm looking for a cheap solution for keeping the water fresh (if there is such a thing as a cheap solution!) I'm thinking of trying to rig up a maxi jet 1200 inside of a make-shift basket and just sit it down in the pond, attach a plastic tube which would push the water up and out in a "fountain" style, to move the water and aerate the pond a bit. Would that be enough to keep the pond from going sour? If not, can anyone come up with a simple and cheap solution for keeping the water fresh so I don't have to empty and refill it so often? The ducks won't even get in to swim or drink after about 24 hours.

That pretty much the arrangement we use on our pond. It's about 300G and lightly stocked though. They do make pressurized pond filters that are just basically external canisters that could help with higher waste levels.

Don't a lot of people use UV Sterilizers on their ponds?

what about a 25-30W system connected to that 1200 maxijet you were referring to?

Cheapest solution would be to use one of the cheap internal filters.