If you fill one grow bed, and then the other in turn you will remove less water from the fish tank at any one time as there will always be another garden bed draining water back into it. There are a few products you can buy to achieve this, but pictured here is my first attempt at in invention to make this occur. Basically its a sea-saw where water is diverted from one side to the other when a float raises to the point where the sea-saw tips, diverting water to the other side. The sea-saw's floats are set at just the right height so that it tips to the other side just before the auto-siphons trigger. The siphons are triggered by the other smaller flow (water flows are all constantly on, and are pictured here as the blue arrows pointing down from the top) I made one of these and it seemed reliable enough even though it looked a bit like it was made by Homer Simpson.
I don't intend to use a sequencer in my system as I'm going to use CHIFT PIST (Constant height in fish tank, pump in sump tank) which means there will be plenty of water for everything. If space is a problem a sequencer may be the solution. Aquaponics often involves compromise when building a system, especially on a budget, so I thought I would post this just in case it is useful.