draining ebb and flow

[Wade]

If I am understanding the ebb and flow process right the tube from the pump that fills the tray is also the drain tube back to the reservoir. Sounds simple but does this back flow cause problems with the pump over time? Would this action tend to back flush the pump filter and keep it clean? How do you determine the size of pump that you need like for a 20 gal. tray or 4 five gallon buckets? How quick do you want to fill them to the point of overflow? And last of all how long do you want it to soak before it drains? I read everything from a max. of 15 minutes to a quick fill and drain. Thanks again.