I am looking into getting an ATO for a 75 display with 20 sump. I would like to go with Tunze since it seems to be the most trusted brand for ATO across all forums that I lurk. My big question is how are they sized? I see the Tunze Nano is sized for "Up to 50 gallons" but why would it matter to the ATO if the tank is actually 75? I ask because the Nano seems to be about half of the price of the other options and am just curious what the difference is. I understand it probably would end up over worked for a much larger tank(into the hundreds of gallons) but only up to a 75 isn't that big of a jump right?