IME they are a waste of money unless you are trying to eliminate water born algaes and/or "green water." Since their usefulness is bound by what is taken from the water column, most parasites are just retarded in numbers at best, but not necessarily killed off. On the plus side, uv
sterilizers will kill unwanted bacteria, but then again how can you honestly tell the difference between unwanted and wanted bacteria, which the latter is also killed along with the pods
, worms, etc that Ziggy mentioned. At best, I can only see their usefulness to breeders who would like to kill any unwanted micro/organisms as a failsafe to their brood stocks.