what is the best flow rate for pond UV water filters?

Answer 1

For good efficiency, you'll want to go with around 20 gallons per hour per watt of UV (or less). So if you have an 40 watt unit, your flow should be no more than 800 GPH.

Since how effective a UV will be is related to exposure to the UV radiation, the slower the flow, the more efficient it will be at doing what you want it to. Trying to use too fast of a flow (maybe thinking you want to cycle the water through the sterilizer faster so the pond gets treated more quickly) may not provide enough exposure time to the radiation to affect what you're trying to be rid of.