I use my 20W Danner UV with a Magnum 350 using the micron cartdige... excellent results... but this is to eradicate parasites, calrify water, kill algae and all other UV benefits...
Using a little 5~7W w/ built in powerhead won't give you the full range of benefits, but will work like a charm against algae.
You could use the FX5, but you'll wat to T off the main line... You could do this and go with a larger UV, such as a 13~20W light at around 10 gph per W to give yourself the full range of UV benefits...
Or grab one of the little 5 or 7W units with a built in powerhead to take care of the algae only...
There are large UV unuits that can be hooked up to the FX5's full blast, but now your talking high dollar lights that are more high powered than your tank needs...