UV Sterilization is the safest method of preventing and eliminating problems in an aquarium, or pond. UV will not kill a parasite on the fish, but parasites go through a free floating stage, at which point they are eliminated. Additionally, fish can and do carry bacteria which can infect and spread throughout the tank, killing other inhabitants.
Most effective if bulb is new, or replaced regularly (at least every 11 months).
Effectiveness can be hindered if the water passes to fast past the bulb. Most effective if the exposure time of the water to the UV light is longer than one second.