If it's a ready made assembly, there should be a valve between the pump and the venturi. If you partially close that, it will take a bit longer to fill the tank but on my outfit, it seems that more stuff is sucked into the stream for every gallon that goes into the tank. If you play around with the valve, you should find the sweet spot where the venturi has the most pull. On mine it isn't when the valve is wide open.

I'm only filling a 220 gal tank and putting a dose of W.P. SU, 10 gal. of liquid form phenoxy, a glug of surfactant, and triple rinsing the phenoxy jugs. Still have about 50 gal.+ of water to put in after all the good, high $$ stuff has been sucked in.