How many tanks do you have? I would imagine it's much less expensive to heat the tank water than to heat the whole room. We keep our house at 65 in the winter and there's a heater in the tank to bring it to 76. I have a meter and could measure the energy consumption and translate it into dollars to know for sure, but I'm guessing it's negligible compared to how much our heating bill would go up if we heated the whole house at 76.
Of course if you have a "fish room" where you can heat just that room, and you have 20 tanks and don't want 20 individual heaters running, then it might make more sense to heat the room.