I think if you can get the flow rate down, that the 9W would work. I don't know if that's best done before or after the canister, but after seems like the best way. (A T-valve? I'm not a plumber, so don't think I can help with that much.)
The next size up (15W?) will be stronger (and work at a faster gph) but the contact time will still be to short to work at the recommended rate. The gph for parasites is about 5x wattage, (even assuming the fluval 206 is actually running lower at say 120 gph when it's a while between cleaning), so would imply around a 25W bulb. Diverting part of the output would seem the best choice for a 9W, or even a 15W.
And yea, I saw a few posts about live worms. Bah! I love live feeding food, but frozen seems so much safer.