Have a 3 gallon Critter Keeper that's hot pink, I want to put one of my female bettas that are in the 1.5 gallon in it but how can I safely put a heater in it without accidently melting the tank. I only paid a 1.00 for it at a yard sale last year so I hate to have to buy a tank for 20 some bucks where I could use this instead.