I have never heard of any fuse that is not even numbered for AC current. I would think no matter how small your pump is, the fuses should be be at least 15 amp. When pump motors start they pull 3 to 5 times the amperage they normally run at. If the pump is starting and stopping a lot, this alone could blow a fuse.
JoeD is correct, we need to know more about your pump motor to get any closer to what you need.