Originally Posted by
catfish whiskers
What I believe occurred , is that the upper unit was air locked , and all or most of the flow passed through only the lower unit . When the units are opened to change the bulbs , and then the pump is then turned back on , air could possibly rise and get trapped in the upper unit . With the upper unit mounted the way it is , there is no way to bleed off any trapped air . If You want to keep them both mounted horizontally, redo the top unit so that the union’s are facing up just like the lower unit , that way they will both have water flow even if the flow is uneven .
Turning the entire assembly vertical with the inlets at the bottom would also be good .
Their multi bulb units , although all running the bulbs in series , all have a single inlet and single outlet , and that forces water evenly through the entire unit .