Hello,
I gather from somewhere that there is a restrictor within RA680 to control the flow at 1200LPH. Regardless on how high the flowrate at the input to chiller, the output remains.
Looking at your drawing, from output of chiller to tank is around 11feet. Hence, with 1200LPH at output, it's not surprising the flow becomes like pee at the maintank.
Do not overlook the heat loss (or rather loss of coolness) for that 11feet pipe.
My 2 cents.