MorganS:
First: congratulations for using the schematic view!The second circuit is correct. The Mosfet functions with voltage differences so you need to connect to both sides to make a voltage.
However the power supply must have some other internal connection which is bad for the USB. Can you unplug your laptop and run on battery power while connected to this circuit?
I already tried that, unfortunately didn't help...
But I'm planning on powering the arduino with another source when the Project is ready, so that would probably solve the problem?
avr_fred:
What exactly is an “adjustable power adapter”? Please explain/provide details as that is likely to be the source of the problem.
Something like this
outsider:
And you don't show a free wheeling diode across the pump, you need that to protect the MOSFET and helps with noise suppression.
I have one, just forgot to include in the sketch