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An aquarium UV steriliser should be switched on and running for 24 hours per day, every day. Exceptions would be setting the tank up before there are any fish in it, adding beneficial bacteria to the water, as UV light kills bacteria, or if you are using a medication that stipulates that UVs should be turned off.
Some marine keepers like to run UV intermittently to allow phytoplankton and zooplankton a chance to feed and travel through the system without getting zapped, but this also allows parasites to do the same, so is non-sensical if you installed a UV on the system to help protect your fish from parasites with free-swimming stages.
But when running a UV for 24 hours per day, make a record of when it was fitted, as the bulbs will need to be changed every 6 or 12 months, depending on the bulb brand and the manufacturer’s recommendations. Some fish wholesalers and retailers routinely change all their UV tubes every six months, to ensure that they are always performing at their optimum.
When changing a bulb it’s also important to clean the quartz sleeve as its a clear barrier between the light and the water, and if it gets covered in limescale or dirt, the light cannot penetrate as effectively.
Increase contact time
Most aquarium owners run too much water, too quickly through their UV sterilisers. With UV its all about contact time so to be effective on unicellular algae or large parasites the water should pass close to the bulb, and flow along the entire length of the bulb slowly.
The more time spent next to the UV rays, the more effective they are at damaging anything that flows past them. If you have a powerful pump and have no choice but to put 1000lph plus through an aquarium UV steriliser, always choose the largest model and the highest rating in watts, that you can.