(Oh sure, I step out for a 2 hour lunch and...
)
The important part is keeping the res inlet and outlet submerged, i.e. under the water level. Badly setup, you might have a vortex that sucks air in the res outlet (the outlet of the res should be at the bottom).
Also, you might consider using clamps on the tubing, especially around the pump inlet.
We have very few details of the setup. A pic would help. Can you see air bubbles in your tubing? DO you use some kind of airtrap?
PS: Welcome to ProCooling!