Assuming that you disassembled both the UZ sterilizer and the pump prior to installation (to make sure there is no packing material or other foreign objects inside them), I think the problem is with your pump.
I can't find any formal specifications for that pump online other than what you included in your original post (317 gah and 50" max head), but just those two data points sound an alarm bell for me.
As an actual pump engineer myself (I work in the Oil & gas industry and deal with these kinds of things all the time ... just on a much larger scale), here's what those two data points are saying:
1) If you installed that pump in a body of water (so that the inlet was submerged) but didn't attach ANYTHING to the discharge pipe, you should be able to measure 317 gallons per hour out of the discharge in the vertical direction
2) if you now install a vertical pipe to the discharge of that pump that is the same diameter as the discharge itself, for every inch of vertical length added to that pipe, the flow rate of the pump will decrease non-linearly
3) once that pipe was more than 50" above the surface of the water at the inlet, there would be NO water exiting the pipe.
In other words, this pump produces such little discharge pressure than anything more challenging that a straight pipe 50" vertical would stop all flow from the pump.
Now ... I know you are thinking that you are not installing the UV sterilizer vertically and that you certainly aren't using 6 feet of pipe to do it ... but actually you are! Every time the water has to flow through a fitting, a piece straight pipe, or any device (such as the UV sterilizer) ... even if they are all horizontal ... you actually create an "equivalent" increase in static vertical head. And this increase in static vertical head is actually greater for flexible tubing than it is for rigid pipe ... and it is different for PVC than it is for silicon (for example). So, given the spiral design of this sterilizer (which actually increases the length of the path that the water has to flow when moving through it), and the need to hose a bunch of barbed hose fittings and flexible tube to plumb it all together, I would not be surprised AT ALL to learn that you have added enough static head to plumbing that the resulting in very little flow!
As it turns out, I think you've effectively demonstrated that the plumbing you are using has added 50" of static head to the pump!
So ... guy buy yourself a bigger pump.
When you do, look for a pump that provides a "Pump Curve" on the box or in the manual that shows you the shape of the curve between head and flow ... then find one that give you 200 GPH at 50 inches (6 feet) of head.
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