I had some fancy guppy's in my front puddle one year. Started out with 2 pairs, when fall came I had probably close to 200 babies. Let me tell you, that was alot of fun trying to catch them all.
I've also kept a pleco in my Wakin/Shubunkin pond. I called it his summer home, he was 19 1/2 and still hard to catch at the end of the season. We had him many, many years and lost him last year. Still haven't bought another one yet. They say the will suck the slime coating off of goldfish, I never had a problem with him though.
Kim