High fhydro,
Since all the buckets are connected to the control bucket,all the emptying and filling happens thru the control bucket.
Once a week,when its time to change the nutes,you simply put a general utility pump in your control bucket and pump all the nutes out of the control bucket for disposal.(down the drain,in your garden,whatever)
Once the nutes have been pumped out you put your pump in your rez and you pump from the rez to the control bucket and all the buckets will fill at same time as the control bucket.
You stop pumping when you reach the desired level in your control bucket.(remember,all buckets will have same liquid level as the control bucket)
As a rez,i use a 45 gallon drum because i have to mix 160 litres every week for the nute change.
I dont use recirculation as i feel its only a waste of energy and my gilrs dont complain either...lol
I adjust ph when i fill the rez then add nutes and then i dont bother with it anymore since nutes are changed weekly.

QB