Different UV sterilizers have different power when it comes to killing micro-organisms (bacteria and viruses). Usually, the most affordable UV sterilizers have been designed to help in killing algae but they don’t have any effects of viruses and bacteria.
The Level 1 UV sterilizers are very powerful, and they have been designed to kill viruses and bacteria. Most aquariums tend to use them since they are very effective, and they operate at a high flow rate and clean the tank faster.
On the other hand, Level 2 UV sterilizers are very powerful for home-application. Unlike Level 1 UV sterilizers, Level 2 works at a very low flow rate, and they tend to have more UV light concentration. This makes them perfect for use in getting rid of viruses and multicellular parasites that might affect your fish.