Yes... I would ask him because it does not add up. Your $90 a month sounds about right. I am usually in Tier 4 (~$0.37) within 2 weeks of a billing cycle (Big house, pool, etc.) and when I was running metal halide on my 300g, the tank was costing me just over $100 per month in electricity. So what your landlord is suggesting make no sense.
BTW, I would be willing to take all your fish if you go down that route.