White cloud mountain minnow
As we all know there are fish which we regard as being cold water fish and there are fish which we regard as being tropical fish. But in nature the world doesn't suddenly go from cold to tropical the changes occurs very gradually. Most fish live somewhere between the two but this isn't often reflected in the way that we keep our fish.
Cypriniformes - Carps
Siluriformes - Catfish
Cyprinodontiformes - Livebearers and killifish
Perciformes - Perches
Obviously there are many thousand more fish which fit in to this category and most of these fish will do better and live much longer if they are kept at a lower temperature than is normally found in most tropical aquariums. It is very easy to research the needs of most species of fish apart from simply asking the vendor who will likely as not tell you what he needs to in order to make a sale.
If you buy one of the fish which normally does best at lower temperatures from a shop where it has been kept at higher temps, don't suddenly place the fish in cooler water. The chances are it will become lethargic and go off its food. It is better to slower acclimatise the fish over a period of days slowly lowering the temp by a couple of degrees at a time.
A heater should still be included because these fish still have a minimum temperature requirement and cool nights when the central heating is off could see the temp dropping below what is ideal. Apart from that temperature fluctuations are generally stressfull to the fish and should be avoided.