you have to have a big enough reservoir to allow the tray to be flooded and start to drain back to rez before res level gets below pump. does this answer your question? If you are still queried, then search ebb and flow, and check out the fill/drain fittings. the tall one is the drain.
researchn what size reservoirs are used with a certain model tray. usually 40 gallons for the 3x3 trays I think. peace