What are "cold water" and "warm water"?
Which temperature range is "cold water", and which one is "warm water"?
this question is not as simple as it may look at first!
I will start with the most clearly defined section - warm water: This (concerning the fish) refers to species from the tropics and the subtropics, this includes
* on the American continents: roughly from northern Argentina to the southern USA (Texas, Florida). The largest number of fish species originates from northern South America (tetras, catfish, cichlids) and Central America (livebearers, cichlids).
* on the African continent: regions south of the Sahara desert to, roughly, Mozambique (Lake Malawi!)
* on the Asian continent: India, Southeast Asia (Thailand etc.), southern China
* Indonesia/Australia: all.
The second group is the one that is mostly/usually called 'coldwater species', which is not entirely correct from a strict viewpoint. This group includes all fish that feel well in not separately heated indoor aquariums with approximately 18 - 22 °C and partially (goldfish with normal body shape!) can be kept even at lower temperatures. For instance, White Cloud Mountain Minnows (Tanichthys albonubes) from China or some hillstream loaches, also from China, are suitable for non-heated indoor aquariums at around 20 °C. These species have in common that they can be kept at these temperatures throughout the year.
The third group consists of real coldwater fish, which are not very commonly kept in aquariums due to the required efforts. These fish (e.g. sticklebacks or some Sunfish species (Elassoma spp., Pygmy Sunfish) have a rather low upper (!) temperature limit, typically below 20 °C, that must not be exceeded for a long time. Many of these species must hibernate at cold temperatures (i.e. below approximately 8 °C), otherwise they become sensitive to diseases and/or lose their fertility. Generally, sufficiently deep (to prevent them freezing over!) ponds ponds are best suited for these fish; when keeping them indoors they must (!) be transferred into an aptly cold environment during the winter.
Dr. Bodo Schnell