Our power bill dropped about $50 a month in winter after installing the heat pump as well. It did go up about $20 a month in summer though...
I used to have 42 tanks totaling nearly 6000L, 3 inside (1200L, 700L and 550L) as display tanks and the rest in a well insulated room in the garage. I had a central filter running a 350W pump, 200W of lighting and 800W of heating in the fishroom. There was also a continuous 150L/day waterchange going on. Inside, 30% water changes once a week. All these tanks with just me on my own meant a $225 - $250 power bill (and I had 2 computers running 24/7 as well).
Some of the power bills mentioned indicate a lot of waste going on. I felt guilty using the power I did back then...