I buy some minnows from a bait store every few weeks when I go to the feed store and throw them in the ponds when I get home (I love the little old man who owns and runs the place so stop and buy them just to visit with him).
Our ponds aren't very old, one is 2 years and the other 1 year and we didn't stock them: Didn't figure fish would stand a chance against the ducks. We have found accidental fish in there (a catfish and sunfish): I have no idea how they got there unless they hitched a ride with some of the herons that visit.
We always have a ton of tadpoles though.
Anyway, by raising minnows in the aquaponics system, I get my plants fertilized and get to scoop some out and replenish the pond population every couple weeks. I was growing duck weed, but couldn't keep up with demand...greedy little ducks!