You have to do something to let water drain from the grow beds.
If I wanted to just have a simple flood and drain system, I could pump water in the grow bed for a while, then let the water gush or dribble out a hole in the bottom.
But as soon as I want to control that water in any way, I need to install a bulkhead.
The bulkhead I use for the 365 Aquaponics System is constructed from inexpensive PVC pieces you can buy at any hardware store. I use ¾” PVC pipe and fittings. If using metric plumbing bits, the equivalent size is 19 mm.
For some inexplicable reason, plumbing bits in the US are designed with a rounded edge. So for the actual part that penetrates the tank, I use PVC bits designed for electrical conduit.
The male PVC coupling bit is screwed down through the thickness of the tank. Once the coupling is tight, PVC will seat itself against the plastic tank wall in a nearly water-tight fashion. Slip a #18 O-ring around the male threads, then thread the female fitting as tight as you can by hand. The O-ring will make this bulkhead water tight, given that none of these bulkheads needs to withstand more than a foot or 300mm of water pressure.
Credit for this bulkhead concept goes to Richard Kinch, who details this bulkhead design at "An Improvised PVC Bulkhead Fitting for Liquid Storage Tanks."
Here’s a short video clip showing installation of a bulkhead.