I think there is a misunderstanding in that folk think pond volume determines the UV size and wattage. This is not correct.
Pond flow rate is what determines the UV's size and wattage and effectiveness.
Flow rate alone does not determine if the UV is a sterilizer. The strength of the UV light spectrum is what matters when talking about being a sterilizer to kill pathogens, virus, and parasites. Here is a thread, from AquaticEco about sterilizers
, that talks more about what is needed to make a UV into a sterilizer.
The diameter of the UV pipe matters as well. UV light can penetrate water as deep as three inches. So, eventhough a 1" flow of water through the UV works fine, you will get the most bang for your buck with a 3" flow of water through the UV.
How do I choose my UV ??
This is a tough subject since there is no easy way to talk flow rates without metering the water, but you can take a rough guess by looking at your pump's flow rate, do some head/friction loss calculations, and then determine your actual flow rate. When properly installed, the single cell pea soup algae should clear within around a week or sooner; so, if this has not been achieved, then slow the flow rate of water with a valve prior to the UV.
For the UV to be a sterilizer with the proper light strength, then the flow rate is approximately 10~20 gph per UV watt rating. So, for a 9 watt UV to be a sterilizer, assuming it has not lost any of its light strength, then the flow rate should be around 90~180gph through this UV.
UV is a sterilizer when it attempts to obtain a 90~100% kill rate of everything in the water that passes by the light.
UV is a clarifier whenever it obtains a much lower kill rate. A kill rate just long enough to kill the single cell algae and I don't know what this kill rate would be to accomplish this.
From reading folk's testimonials, it seems a flow rate of anywhere between 50~100 gph per UV watt rating is enough, but, ultimately, this is told to you by the manufactuer's product flow chart. Although, I have seen the flow rate being anywhere from 40~120gph per UV watt rate, but some folk seem to do just fine with a higher flow rate although it might take longer to obtain the results they desire.
The two brands I highly recommend, due to other professionals that I know who use them, are Evolution Aqua evoUV
and Emperor Aquatics
. Often times, the more profesional pond hobbyists will have multiple UVs, either multiple 55watt or multiple 110watt, units in parallel so they can significantly improve their flow rate.
Ultimately, choosing your UV depends on your flow rate and what type of flow rate you want to have while effectively using a UV.