If your apartment was cold this may be the reason the fish died. However, my home has temps that fluctuate so I have 2 nice sized goldfish that I have had for 2 yrs., that live in a fish tank without a heater but I do have a filter to keep the water clean. My grandson takes care ofaquariums so I consulted him and he said to take half of the water out, leave the rest in because fish need a certain amount of bacteria in the water and if you take it all out it will create a problem and the new fish may die. So, you can get some goldfish which will withstand the cold water and things should be okay. Have the child go with you and pick out the new fish, give them names and it should help him forget about the Betta. I bought my goldfish at Wal-Mart 2 yrs. ago and they're still alive. Don't overfeed them. let me know your results. GOOD LUCK!