A steriliser will help in two main areas - as a clarifier and for micro organisms. As a clarifier this will help visually what you see in the aquarium with working on algae and greenwater. Micro-Organisms like bacteria, viruses and protozoa, you will generally not be able to visibly see and helps to prevent these organisms being passed from your aquarium inhabitants like Fish, invertebrates and corals to each other and free swimming in your aquarium.
How effective will a UV Steriliser be?
In order for a steriliser to work there are a number of contributing factors to take into consideration:-
It must be free floating in the water column.
It needs to pass through the UV Steriliser.
The UV needs to be strong enough to radiate UV to kill Viruses, Algae, Bacteria and Protozoa.
There needs to be sufficient dwell time for the organism to be in contact with the UV.
While a rock in your aquarium with algae on it will not be affected by the UV, this is because it is not passing through the UV, the spores that are floating around in your water will be affected as they can pass through the UV similarly with virus, bacteria and protozoa.
Generally the higher the wattage of a steriliser the more light it can produce but keep in mind that the bulb does have a life and generally has a life span between 6-12 months and as it get closer to is end of life of the bulb the emission will also be reduced and regular maintenance is required for effective use.
Dwell time is also a important factor and this is the amount of time that the organism is exposed to the UV. This is a combination of the length of the UV that it has to pass, to which the organism is exposed and the time, if the flow of the unit is too fast then it can result in the steriliser being ineffective.
For example a Eheim Reeflex 11 watt UV Steriliser recommended for a tank between 400-800 litres, it can have a maximum flow rate of 1200 litres per hour as a clarifier and having clear water but that would not allow sufficient dwell time to help with microbes - the flow would need to be a maximum of 600lph to be effective on microbes.
UV and microbes work in way that the UV will mutate the structure preventing them from reproduction but be aware some microbes can be unaffected by UV.