Adding a reservoir

I have a 10 gallon husky pail and I intend on adding a 20 gallon reservoir to it. Should I be using a 1/2 inch or 3/4 barrel bulkhead for this job?

https://www.homedepot.com/p/Everbilt-3-4-in-Barrel-Bulkhead-Union-with-EPDM-Washer-800479/207176307

Would depend on the pump (and you will need one not like the photo showing no pump) but i would not go less than 1" if you plan to upgrade later then take that into account. I use these https://www.amazon.com/Lifegard-Aqu...khead+fitting&qid=1556817088&s=gateway&sr=8-5 There is no real downside to using bigger even 2"

Added: Unless you plan to run a submersible in the bucket running the output hose from the pump through the pipe into the other bucket. You would still need to size on GPH and account for possibly 1/2" hose? loss of size but it would save you the cost of 2 more bulkheads i guess. If you consider that a benefit.

1/2" and a cheap, small water pump will work fine for recirculating your water. But, with only a single plant, it's really not necessary. The water pump's going to be adding heat to the water, which will give you some extra work trying to keep it below 70 (recommended). You'd be better off going with a single 27-gallon HDX tote for your plant, if you have the space.