I agree with Tony on a lot of points
I find running a UV unit is a lot of 'feel good'
I run mine at a low enough flow to be beyond the 'green algae' control and closer to 'bug' control
I only change my bulbs once a year and, with the Pentair Lifeguard unit I own, will need to change the inner sleeve every 2 years or so as UV breaks down plastics
It's still only a 'bandaid' for a lot of issues and is best used as a water clarifier, with it plumbed into your system after your filtration to be the most effective
There's lots of good info on the web pertaining to UV flow rates
I use the info
here for a guide
Specifically, this excerpt:
*As a Clarifier; For Algae Control (Green Water and some Cloudy Water), 40-50 gph per watt is effective to maintain effective exposure for effective UVC sterilization/radiation (depending on model UV’s design).
High Efficiency (High Dwell/Exposure) UV Sterilizers such as the
TMC Pond Advantage with long contact time as per the wattage and lamp can easily run as high as 50-60 gph per watt. The even higher efficiency (& dwell time)
Pro Clear UV 30 can go 70+ gph per watt.
*As a Sterilizer; Generally for bacterial control (& many virus) a flow rate of 20-25 gph per watt (75-95 liters per hour, per watt), sometimes as high as 30-35 gph per watt for high efficiency UV Sterilizers such as the
TMC Vecton UV.
*As a Sterilizer; For single cell parasite control (such as Cryptocaryon) as well as a few “stubborn” viruses, a flow under 10 gph (or even less) is necessary. This is often not 100% for all parasites of this type, so a UV Sterilizer should not be relied on as the sole preventative for these parasites!