Pump does not get enough current via MOSFET
Step (3)after edits. If there's 12V across the opto output, there's about 0V across the 5K gate resistor and the FET is OFF... 

Try again with the optocoupler ON (probably 10mA through its LED. max 2V across the opto, so that Vgs=10V and the FET turns on.