from reading you need to find out the recommended flow, a mj 600 wont do the job for it to work effectively, the higher the flow is for more water clarity, lower flow for killing bacteria
here's what they recommend for the 3x
MAXIMUM FLOW RATE 10 TO 200 GPH UNIT SIZE 11 IN. X 3.75 IN. DIAMETER MAXIMUM TANK SIZE 125 GALLONS WATTS 9 COLOR BLACK
personally I would use a small pump that does under 100 g.p.h. on a small 9 watt uv
if you want a good pump recommendation I would go for a Ehiem hobby pump the 1046 should be fine. You could use the 1048 if theres a lot of flow loss due to head pressure. But Ehiem is reliable and stands the test of time.