in which tank, I was just thinking of buying a coldwater tank and a heater. In my tropical tank at home i have 3 platy (2 female, 1 male, 1 female heavily pregnant)
a tank is a tank. its what you put in it that makes it either tropical or cold water.
the reason i asked what was living in the cold water tank was because if you had goldies or other cfold water fish adding a heater would possibly make them ill