I like the looks of the skippyfilter wifezilla posted. That filter is remarkably similar, in function at least, to the bio-sand filter I have planned for the potable part of our rainwater system.
http://www.biosandfilter.org/biosandfilter/index.php/item/330
Definately more simple and I'd have to see what the water quality is like. Drinkable, I doubt, but close?
Wifezilla, have you had the water in your duck pond tested?
And the skippy lets me have the waterfall I wanted.
I definately don't know enough about ponding to say whether it would work for stock as well, although I don't see why it wouldn't.
Ducks & geese probably would be fine with it, cows may step on/mess up the arrangement around the filter.
The pond/swimming hole I plan on making will also be the main cistern for our rainwater system. I plan on having a smaller tank, enough for 3-4 days, maybe 500 gallon or so, high on a pedestal to provide some gravity feed for water running into the house. There will be an alternating sand filter that feeds into the secondary cistern. It should help tremendously that the water in the main area will be as clear as possible before feeding to the secondary cistern. If I go with this, the water coming in the house will be bio-sand filtered twice.
Whee!
I'm paranoid about snakes in my area aidenbaby. I almost jumped into a nest of water moccasins when I was much younger and it haunts me to this day. Seeing the water so clear is what really turned me on to this.
So lets see, Skippy filter, Huge hole packed down & lined with clay, solar powered water pump, maybe 2, plants for the edges, deck or steps for entrance, pizza & beer for bribery, er labor

I can do this! One step at a time though.