[bertoni]

They do different things. The carbon is a good idea to run as a sort of emergency filter. It will bind organics that can discolor the water, and also heavy metals that are bound to organics. The UV sterilizer would be good for phytoplankton or bacterial blooms. What kind of problem is your tank having, or are you planning for the future? I'd suggest running carbon, even if the water is clear enough as is.