We have a Pentair Aquatics 40 Watt UV sterilizer connected to our FX5. It works beautifully. The wattage of your UV sterilizer should be based on your tank size (we have a 300 gallon tank) Our particular model is very tall (40") so is not suited for under the stand placement. Ours sits on the floor behind the tank where the FX5 is also located. The return line from the FX5 is attached to the UV sterilizer. Another hose is connected to the UV sterilizer to be the new return line to the tank. The FX5 simply pushes the water through the UV sterilizer and back to the tank.
There are other UV sterilizers, such as the Coralife Turbotwist, which can be connected horizontally. There are fishkeepers who really like their Coralife UVs