Block off any hood vents that might allow heat to rise and escape. Newspaper has amazing insulation factors. Wrap the outside of your tank with a thick layer of newspaper to help retain some of the heat. In Your Aquarium Y ou can generate some heat in your aquarium with a hot water source. Anything that does not need electricity to do so will work.
T ake a sturdy plastic container and put hot boiling water into it and cap it tightly.
We used a thoroughly cleaned 1 gallon Clorox jug. Float the hot water jug in your tank water to radiate some heat back into your tank. You can fill the container with whatever type of heated water you want, but removed aquarium water works well. Use common sense on this one. Use water that is safe to put in your tank, in case some of the water leaks out of the container.