If you add benthic copepods or amphipods (bottom living) vs.planktonic (the free swimming type), you will have more luck seeding the sump as they will be less likely to enter into the water column at night. If your sock and skimmer location in the sump is before the area you want to add them to, they have to go a long way to get back and into the sump and you won't loose as many. Since the reproduce every 7 days or so, they compensate for losses. Tisbe and Tigger copepods as well as amphipods would be good for the sump. However, if you are feeding corals or other filter feeders with planktonic pods, you will probably want to wait until it is dark and reduce or turn off your return flow so they stay in the water column longer.