Not sure where you're at as far as which utilities company you'd use. But DTE will let you do an "Analyze my bill" to break down what each approximate charge is for. In my case, last month was around $35 for the 'other' category which should be computer, TV, and powering the fish tank stuff. I'm running 4x Whisper 60s and 3x AC110s, plus 2x 250W Aqueon Pros, 3 200W Aqueon Pros, a 150W some other brand, and 4-5 50 or 75W heaters. So I would think mine would be much higher than yours, if it was the fish equipment causing the jump.
For example, an AC110 will use 13W max * 24 hours/day * 30 days ina month = ~10kw/month. Which in my case was around 1-1.5% of my bill.
Even leaving every light on would be iffy for that big a jump, IMO. I had around $20 for lighting which would include everything in the house. But I am running all T8s except one 24" fixture.
There is also hot water factored into the electric bill if you have an electric hot water heater, so if you're doing a ton of water changes that could impact, but even so, $120 seems high. I'd call and see if they misread your meter. They did that to me over the summer and overcharged $150.
