when it comes to subtropical fish then only balzanii and other types of gymnogeophagus and peppered cory will survive in 0-5 celsius WATER temperature. IF there is minimum AIR temperature 5 celsius not WATER then i think texas cichlid and tilapia can make it for winter seasson. We must separate two things ,air temperature and water temperature , when air temperature is 5 celsius then water temperature at bottom of pond will be around 8 but only if you pond is deep enough - depth 1 metre and more. It wont work in shallow pond.