UV sterilizers can accomplish clarification, Level One Sterilization, or Level Two Sterilization. Most of us strive to attain Level One Sterilization.
This article has changed over the years. it now breaks down UV sterilizers into different types.
http://www.americanaquariumproducts.com/AquariumUVSterilization.html
Basically, you can use your FX-6, but to do so, you will have to increase the wattage of the UV sterilizer to achieve Level One Sterilization. Depending on the UV sterilizer, you will probably need 36 - 40 Watts minimum.
This is the 40 W UV sterilizer I use with my FX-5 on my 300 gallon system. In your case, it would be more practical to go with a lower flow rate and less wattage.
http://www.marinedepot.com/Lifegard...ifegard_Aquatics-RB5171-FIUVUU-RB5175-vi.html
To achieve Level One Sterilization in a 180 gallon tank, with a flow rate of 270 gph, you would need 13.5 - 18 Watts. At a minimum
\180 gph, 9 - 12 Watts.