Aquaponics Sump tank, Why?
Either to deal with water level fluctuations or gravity drains or both.
So once you decide you need a sump tank, you then need to figure out how big it needs to be.
Start out by figuring out the total volume of all the flood and drain Media grow beds you need to handle flooding and draining at the same time. Then you figure that your media will displace about half the volume of those beds (it will actually displace more but we want to have some extra sump volume so go with 50% since it makes the math easier anyway. Us some volume calculator on the web if you need to figure from cubic inches or feed into gallons or liters and then figure out the total volume needed.
So if you have 1000 gallons of grow beds total and figure that leaves needing about 500 gallons of sump tank volume to handle flooding the beds all at once, then you should look for a sump tank that will hold at least 500 gallon but I often recommend going about 10% extra just so you have some added depth for your pump to operate (most can’t pump a tank down to dry ya know) and it’s kinda handy to also install a float controlled top up valve just below the normal low water level but above the pump overheating or sucking air level.
This calculation works well in basic systems but keep in mind that a nearly empty sump tank sunk into the ground in a greenhouse while there is a downpour all around might have a tendency to float out of the hole, this can do quite a bit of damage to the plumbing and the hole as well as create a HSM (heart stopping moment) when the owner of the aquaponics system discovers the damage. Also very shallow but large footprint sumps might be easier to fit under grow beds without digging them into a hole but they will also not provide much depth for the pump to work in and there for may need to be even larger to accommodate the operational depth of most pumps.