UV sterilizer is not meant to create a completely sterile environment. Most people always have this wrong perception because only the water that passes through the UV lamp gets treated and the number of bacteria reduced. It is impossible to ensure all the bacteria is destroyed because there will always be some present in the aquarium water filter, aquarium plants, gravel bed and decorations.
The purpose of reducing the bacteria load in an aquarium is to give your fish a better chance of survival in case disease breaks out. It is mainly used to control spread of disease such as ich not by completely destroying the parasites but to reduce the population so that effected fish can fight off the disease and ensuring also that other fish will not succumb to the same condition. (Note that ich is very hard to contain and it is normal to find the parasite present even on the fish skin surface)
The most common method in choosing the right UV sterilizer is by taking into consideration of the microwatts/cm2 value and usually a benchmark figure is about 28 000 for it to be effective. You might want to consider also on the length of time for the exposure (measured in seconds) on the amount of water which your UV unit can hold. Don’t get confused by all these numbers here because basically what you need is your aquarium tank capacity and the salesperson might be able to provide a good recommendation for your requirements.