greenhouse heating

[NATURALIS]

Hi,All, as the weather is changing early this year i have been looking into ways to increase the temps in the greenhouse,i recently read an article about burying a plastic container filling it with water with a lid in the ground with a fish tank heater, to create hot spots in the ground. I wondered if anyone else has tried this?. also what is the best material to use on the walls and roof of a green house to retain more of the heat, i thought about bubble wrap, any other ideas please.