Guppies are able to deal with lower temperatures than most tropical fishes, as are some barbs (like rosys, golden etc) and other fish. They are fine in "cold" water if the temperature is not less than 18C. The average well insulated house in many parts of NZ would not get below this, hence the term "coldwater" means "in a well insulated house in a warm part of the country so you do not require a heaterstat" 8)