I would put a bigger one or two of them on a tank that size. Is it a 75, 90? If it's a 55, you would probably be ok, but anything larger, I'd go bigger. It's rated at 180 gph or up to a 40 gallon tank, so you'd be lucky to be turning a 55 gallon over three times per hour.
ok thanks for the advice i have a ehiem, (but im not sure what size it is and i dont really know how many gallons lol it a 4x1.5x1 so im not sure), i could prob run that with the fluval also and that be ok, its only temp untill i get my indoor pond made after the year. thanks again dudes