A ball valve would cut down the gph
from the pump, yes, but now you're getting into a T in the return line and it seems like the water would take the easiest path back to the tank, which would essentially bypass that split altogether? Kinda confusing but makes sense in my twisted mind lol... Closed loop- basically means the water is pump fed, no gravity involved, no overflow, etc... for example in one of your chambers, just run a pump with a line going into the reactor, and then the return from the reactor right back into the same chamber, or I would imagine you could dump it in the next chamber if
you wanted. Some people even just put the reactor right in the sump.