Whatever you put in, better put in a heater in the winter, or it'll freeze solid.
Down here the problem is heat.
Over time a sufficiently deep (at least three feet) garden pond will assume the average air temperature of the setting, which in July and August around here is about 85F.
Goldfish are remarkably tough fish, actually cold water but obviously can survive considerable heat.
OTOH our topminnows got so out of control I put in an oscar. That guy got big and survived until late November, when the average temps got down below 60.
