What are UV sterilisers and what do they do?
Removing bad or harmful microbiota (bacteria, archaea, protists, fungi and viruses) from your aquarium can be a constant battle. Aquariums often provide the ideal breeding grounds for such organisms to spread quickly, so if you sit back and do nothing you may have a big problem on your hands. UV sterilisation units that are correctly fitted will kill free floating microbiota therefore i strongly advise you get one for your aquarium. UV radiation is undoubtedly the smart way to control pathogenicity and water clarity. I know for a fact that most of the aquatic professionals that I know, run with UV lighting systems.
If you have a tropical freshwater setup then you may not need a UV setup, however saltwater aquariums should have one installed. That said i do believe all types of aquariums would benefit from UV lights.
How long do they last?
As you may have guessed, UV light is only as good as its lamp intensity. This means that bulbs should be replaced periodically as the intensity starts to weaken. The frequency at which you must change these bulbs will depend on the manufacturers recommendations but is is fairly common to change these once every 6 months. Most good units have lights that tell the user the condition of the bulb. Green indicates everything is ok and red means you need to change the bulb.
What type of UV steriliser should I buy and where should I place it in my aquarium?
You need to purchase a UV light unit that matches your flow rate and water volume. Usually you can check on the packaging. As for placement, I in fact, place my UV unit in the flow return pipe. Always fit the UV unit at the very last stage of filtration and just before the water enters back into the aquarium.
What is the difference between in-line or hang-on UV sterilisers?.
As their name suggests, in-line models are plumbed directly into the main aquarium filtration system. The UV is placed after the mechanical filtration unit and should be the last in-line device before water returns to the aquarium.
These compact sterilizers are mounted directly to the back of the aquarium. They are generally used as an independent device fed by a submerged powerhead. However, some hang-on sterilizers may also be connected to the return line from a canister filter or in-line filtration systems. Hang-on models tend to be easier to install and maintain, making them ideal for smaller aquarium setups.
Make sure you choose the right unit for you.
Proper flow rate through the uv unit determines how effective it is at killing various organisms. For example parasites are typically larger than bacteria and therefore require longer exposure to uv to kill it. This can be achieved by simply reducing the flow rate which increases the exposure time to uv. Good sterilisers allow you to make these adjustments.
As a rough guide 8 an watt bulb will control bacteria and algae at a flow rate of 120 gallons per hour. A 15 watt bulb will control bacteria and algae at a flow rate of 230 gallons per hour. But to kill Parasites a reduced flow rate of 75 gallons per hour will be required.