if you cant afford a new heater (which is what I recommend, get a jager), then insulate the tank as best as you can with a towel or blanket as posted above, you could also leave your lights on to help with heat, but you might have algae issues because of it.
I think your best plan of action though is to get a good heater and when they replace the other one with a new one, sell it.
I think your best plan of action though is to get a good heater and when they replace the other one with a new one, sell it.