That happened to me last year.Both of my pelcos went missing and when we took the plants out of the pond we found them frozen
.But all of our goldfish lived for 5 years,and the only reason that they died was because a bird ate them
.Also,we never had a heater or any of that fancy stuff
.All we had was a filter.We would add some alge preventer that my mom had and the alge never got to bad.The fish got huge and seemed happy,then they were eaten
.We have had a pond for years and we have never had a problem with it getting too dirty.