This is eight ounces of water we're talking about, and the amount of energy required to heat a volume of material to a certain point has to take into account the heat transfer characteristics of the materials and the environment involved. That being said even under optimal conditions 2.5watts for maintaining that temperture in a normal environment is probably not gonna happen. I'd guess close to 7-10 watts. So you either need tot use more USB ports, or do something inteligent and supply it power external to the system. The 'cool' factor of a usb powered fish tank is totally shot down by practical factor that it's going to end up stressing a power supply designed to power data devices....