I want to make everyone aware that those inexpensive solenoid valves like used for irrigation are not necessarily the simple way to control water flow to grow beds.
See those solenoid valve require a certain amount of water pressure to operate properly. If there isn't enough pressure behind them, they may not open to let water flow through at all. Very few aquaponics systems are likely to run pumps at high enough pressure to effectively operate these valves.
For example: many of these valves require 6 psi of water pressure.
That is equal to 166 inches of water column or 13.8 feet. If your pump can provide a huge flow of water at 13.8 feet of head, then you might still get enough flow through the valve to actually do something with it. However, many small and medium aquaponic systems are running pumps that can barely pump to 14 feet, let alone flood a grow bed in 15 minutes or less while also pumping against the equivalent pressure of that height.
Once you start figuring out how much pump is needed to provide that kind of pressure and flow, it suddenly doesn't seem so efficient to use the cheap solenoid valves.
A note about how the irrigation solenoid valves actually work. (Please forgive me if I get this a little wrong, I'm just trying to explain in sort of layman's terms, I'm not an engineer or physicist and will probably use some not so right terms but hopefully it will shed a little light on the subject.) They are not mechanically opening and closing a path for water. All the solenoid is doing is pulling a plunger back off a tiny hole that allows air pressure to escape from behind a diaphragm. The air pressure being trapped behind the diaphragm is what stops the water from pushing past the diaphram and spring that are actually stopping it.
There are solenoid valves that work differently than that but most of them are either very small, made of metal or not inexpensive enough for our purposes.