I keep fish in plastic storage bins all the time. Don't worry, a heater will not melt the plastic. Even though they can get hot, the heat is constantly being dispersed and cooled by the water circulating around it. They certainly don't get hot enough to melt the plastic--if they did, there would be huge problems with having the very popular acrylic tanks. :)
You should post a new thread about your whole setup and how you plan to proceed with breeding. We could probably help you out a bit.
Last edited by Adastra; 07-26-2010 at 12:33 PM.