I am currently waiting on a replacement heater as mine crapped out, i had to move my platies into a tiny little tank, i put the tank in a polystyrene box and filled the sides with cloth, i put a little filter in and hoped for the best, the temp is hovering around 72F, i know this is more sub-tropical than cold but so far they look ok - in a warm house tank water should level out at a similar temp. I'm just making a point here but I'm not saying to do this, life will be easier for you and much happier for the platys with a heater.