empty the top rez then lift the controller higher then the buckets to flush all the nute into the buckets. keep a eye on the buckets so you don't overflow them.

i have all 8 hooked up and have no overflow problems but you may with only 6. if the plants don't need all the extra mix then don't even use the top rez only add it when the plants actually need it.

that should help for now, but in the long run you should look into making it recirculate the nutes by adding a pump in the controller with a manifold that pumps into the tops of the waterfarms by pushing the manifold tubes through the stones and top part into the lower rez of the farms.