It sounds like these generators, apart from the expensive ones, only run small amounts of wattage. I still maintain that the best ones to go for are the ones that run off gas, they seem to run longer on a small amount of fuel.
The below link is one i found on EBay for $450, it runs for 6.4hrs on every 1.2 gallons
Average cost of gas per gallon = $3.80
Minutes per Gallon ((6.4*60)/1.2) = 320
Cost per minute = $0.0118
Thats very cheap i think, if you had an outage for a day, it would cost around $16 to run the generator. This isnt a bad price to ensure your fish dont die