you could get a plecostomus, those are super easy and and they love algae which you should have a lot of in a pond type setting so your pleco will be happy and get nice and big. You should also look into getting some sort of bottom feeder like a catfish because goldfish can be very messy. This website has a wonderful fish compatability chart:
http://www.liveaquaria.com/general/f...lity_chart.cfm
Check out this website for some ideas:
http://www.liveaquaria.com/product/p...fm?pCatId=1105
I personally love turtles and want a turtle tank really bad but I heard they can be very mean with fish.