Floating water bottles does little to raise the temp, but it can hold the temp steady (I found this out yesterday when I lost power). Boiling tank water, or RO
water to top off is your best bet, just be *super* careful when adding it. Depending on the size of the tank, you may not need to boil it, just bring it up to 90-100F so that the tank temp increases slowly.
Heaters kill UPS batteries, so either an inverter for the heaters, or a generator, are the only things that will keep a tank alive through the long run.