I have the double helix (like Joe) but, 36 watt by Custom Sealife.
Before installing it, my water parameters were fine, but the tank would continually suffer from bacteria / algae blooms.
After installing the UV, I have had crystal clear water ever since. That is the NUMBER ONE reason I have it.
The U.V. will kill (if properly installed) all free-floating bacteria and algae thus, no more bacteria / algae blooms.
For heavy bioload tanks (like my stingray tank), I think a U.V. is a definite plus for the filtration system. Many ray owners suffer from the blooms and are now considering U.V.s.
And, audio:
I have (like I replied in the other forum) the same w/d filter but, I use 3 Rio 2500 pumps. Two of them feed from the sump and go directly to the tank. The third pump feeds from the sump, then into the U.V. and then to the tank. I have the U.V. hanging (horizontally) under the tank (in the cabinet) right above the w/d.
The water should be mechanically filtered before going into the U.V. (which yours would do since it would go thru the w/d first before reaching your pump).
I guess if I had one pump (like your mag 12), I would set-up two "Y" valves (inbetween the pump and the tank) and run tubing from the first Y to the U.V. and then back to the second Y (and put a one-way valve in there somewhere).
OR, I would put a second (small) pump in the sump, feed the U.V. with this pump and then from the U.V., run a line to connect into the Mag 12's line that is going back to the tank.
OR, same as the second idea, but have the return line from the U.V. end right underneath the Mag 12's IN line so that it would grab that water and shoot it into the tank.
Hope this helped.
Richard.