I use the same size tubing as what the water pump takes (1/2") to connect my three buckets together, they all have overflows to the correct level below the netpots, and one of them has an extra hole for the water pump to pump water in. I dont have a controller reservoir basically, the 50gal reservoir at the bottom is the controller. 300 gph water pump

so basically 1/2" would be fine, or whatever you want to use, as long as it can flow fast enough for your pump