A few wraps of teflon tape and a bead of silicone over that usually seals it well on the first try. Some pumps expand and contract when out of the water so you’ll need many more wraps first; at the pump.
I should have put some teflon on the barb connectors for the return line. I plumped my tank last week and had a very slow drip from where the return tube connects to the overflow box. Used some flex seal and it's good to go.