We have a UV set up on our 6Ft, I only use it when we notice a problem starting or it just dont look right. Leave it on for 48Hr's then turn it off again.
Your right, it will kill good bacteria, in the WATER, so your throwing out good bacteria everytime you do a water change. The bulk of good bac will be in your live rock and filter material, not in the water. I have a mesh around the pump that supply's the UV, this way those little pod's dont get sucked in and the Manderian gets a chrispy meal out the other end :eek:
Just my 2.5 cents worth