I don't know much about this particular product,but I built one years ago and did a lot of reading at the time. There are some very important factors in their operation, such as:
depth of water to penetrate (only effective on shallow depth of water)
Clarity of water (cannot penetrate water with turbidity)
They have an effective operating temperature.
Has to have sufficient contact time to be effective
The bulbs need to be frequently cleaned or UV is blocked off.
Must be shielded from eyes (damaging to eyesite)
People sometimes forget that there are billions of different microorganisms out there and 99.99999% are harmless or good. We would not do well in a sterile environment and neither will fish.