There's lots of variability in the rates people pay. For instance I see
@oz saying his peak price in California is near 50c/kw whereas mine here is 18c/kw at peak. I know in the grand scheme of my bill the aquarium is only a small factor. My electric water heater is absolutely number 1 guzzler followed by my dehumidifier. Yes central air and the oven use a lot of juice but both are intermittent use.
It's a good question though so maybe I'll do some math to try to figure it out.