How many tanks do you have?
I'm asking because if there's any way you are in the market for one (or more), I've been pleasantly surprised at the way Aqueon's Evolve4 holds heat without a heater.
I keep it at about 75F and when I do partial water changes, replace with water at that temp. The way the top goes on these things creates kind of a greenhouse effect, and the temp doesn't even drop at night. (I heat my home at 72F, so I assume that helps somewhat too.)
Every morning when I get up, I check both nanos (they each have a male betta in them), and the temp has held throughout the night.
Sometimes I get the feeling that the whole world is against me, but deep down I know that's not true. Some of the smaller countries are neutral. Robert Orben