This is covered in Chapter 14: Drainage and Chapter 25: Irrigation in the iAVs Handbook
The water needs to drain out, completely and rapidly so there is no standing water.
Every 2 hours during the day, a pump runs for about 15 minutes and 25% of the water in the fish tank runs along the furrows and starts to drain within the first few minutes – which aerates the water back into the fish tank (via gravity) and helps to push any other solids towards the pump.
The water continues to drain out for about 15-30 minutes after the pump is turned off.
The sand is coarse sand – .4mm to 1.2mm – to allow for fast drainage and to create a vacuum effect whereby the oxygen around the rhizosphere (roots) is replaced (atmospheric exchange).
Rita T. Pryce Selected answer as best 1 January 2025