My Mom has used fish in her tank for years, During the winter they live fine with a heater in the tank.
When you put new fish in the tank you are instructed to acclimate the fish to the new water temp by taking the fish in the plastic bags they came in from the store, and setting them in the tank and letting the fish float around the tank. Then after about half hour or so let the fish loose in the tank.
Once, Mom bought several fish and set the 4 bags of fish in the tank, then went off for half an hour to do chores. Her 2 year old draft thought she brought him a new play toy and put it in the water trough. Mom came back and all the bags were ripped open and a dozen dead bitten fish were strewn around the trough.
Moral of the story, don't leave your new fish unattended, with a baby around.