Here's a spec sheet for the GAST 0523.
I believe this pump draws about 1.64 amps during steady-state operations. That's 180 watts/110 volts, but it would draw about 3 to 5 times that at start-up. That would be pretty close to 5 amps at times 3. I bet the 5 amp rating your are seeing is the inrush at start-up OR the protection amperage (what you would fuse it at).
EAT CROW EDIT: Scratch the mumbo-jumbo above regarding 1.64 amps at steady state. Disclaimer: I am a hobby electronics boob. See next post below.
