Backyard Aquaponics
A place to discuss aquaponics

I'm thinking of using a 150L tub to act as an upstairs ST for my dual-level GBs. I've got about 250L of GBs upstairs, 400L downstairs. I'm contemplating pumping the water from the downstairs ST straight up to the upstairs ST, which I think will probably be about 4000L/hr at the given head height (pump is a loaner, not sure of specs).

What I don't know is what sort of overflow I need to ensure the upstairs tub ST doesn't overfill. I'm currently using 20mm poly all round for my irrigation, and 20mm PVC and grey-water pipe for drainage from the GBs. Surely a single 20mm poly overflow isn't going to empty at 4000L/hr, so I don't know how to tackle this.

If my question isn't clear, you may get more understanding from my system (see sig below).

BTW, the main reason for wanting to do this is to utilise the spare 150L tub, maybe with goldies, as a water-feature, and to remove the current multiple bends/t-connections from my pump to the GBs.