sicce 4.0

I have a Sicce Syncra 4.0 pump unopened /unused that I got with my tank bundle from Waterbox. I went with a different pump since it’s a seahorse tank.

All Prices are Shipped Sicce 4.0 Pump - $115 - Rarely used only two or 3 times when setting up large water mix. Sicce .5 - $35 - Works excellent was on previous build no longer used. Hanna Nitrate LR - $50 - BNIB Never used SOLD Nano Flipper Float w/ extra Blades - $35 Red Sea...

I am 100% stumped on this one. I have an Aqua Ultraviolet Advantage 2000 15W with 3/4 barbs and likely 1/2 for the ID on these. I have tried three pumps, Sicce 3.0, 4.0, and SDC 3.0, and none can achieve over 390 GPH. I can tell from the Apex Flow Sensor. Initially, I had the terrible Red Sea...