Nematolebias whitei , N. constanciae , Austrolebias nigripinnis . They're peat divers from South America . I've bred several generations of each in my water (Ph 7.4 , DH, KH 10-13 ) with no problems . Only thing is they don't magically die at the stroke of the new year , but they won't make it to 2 either .
Also note that all three of those species have no need of a heater if the water does not go below the 60's fahrenheit...in fact, Austrolebias sp. in general will do badly if kept above the low 70's fahrenheit for extended periods, and would probably be happy at least down to the low 50's.