Well, UV sterilizers alter the DNA of free-floating pathogens that are in the water. This pretty much kills them. This is for killing bacteria, good and bad, algae, parasites, viruses to some extent, and fungus. It is beneficial to have on large tanks with many fish where these things may become a problem. It is however not a cure for diseases because it only kills things that are free-floating. It also may have an impact on populations of good bacteria and phytoplankton, zooplankton and such. So for most reef tanks uv sterilizers are not used. They are mainly for fish only tanks or holding systems.