I thought that was a very significant consideration. You might just purchase [beg, borrow, steal ;)  ] a likesized heater to the one you have and put it on the other side of the tank. Your filter or airstone should circulate the warm water.

Be aware of the fact that a guppy at 77 F will live longer than one at 80. If you search for temperature and life expectancy you should get something I relayed from Stan Shubel's book on that.

Guppies should be able to drop fry confortably at 76 or 77. Mollies do sometimes need warmer water, less crowding, more veggies and hard (not usually salty) water.