Aquarium UV sterilizers should be on and running 24 hours a day. Exceptions include: setting up the tank before adding fish to it, adding beneficial bacteria to the water (as UV kills bacteria) or if you are using a medication that dictates that the UV must be turned off.
Some marine keepers like to run the UV intermittently to allow phytoplankton and zooplankton a chance to feed and move around the system without being electrocuted, but this also allows parasites to do the same thing, so it is pointless if you have UV on your system to help protect your fish from free-swimming stage parasites.
However, when running the UV light 24 hours a day, keep track of the time it was installed as the bulbs will need to be replaced every 6 or 12 months, depending on the brand of bulb and the manufacturer's recommendations. Some fish wholesalers and retailers will routinely replace all UV tubes every six months to ensure they are always in top condition.
