I have 2 x Pondmaster 40W UV clearing a 12000 USG Koi pond with little shade, and they cleared the water from a "pea-soup" state within a few days. I also use 2 x sand and gravel filters for mechanical filtration, and 2 bio-reactors with 6 cubic feet of K1 aerated; my pond volume only turned over once in 2 hours this past year, but I'll be replacing an older lower-flow pump with a higher-flow pump to reach an hourly turnover of pond volume this coming summer (we winter fish under ice, where I live). This combination kept the water clear, and the ammonia to a level the test kit couldn't detect. I have 27 Koi, half of them less than 3 years old, and the 2 largest over 7 years old. I don't submerse my UVs (I've never been keen on anything electrical submersed into the pond), and they were pump-fed this year, not gravity fed from the bio-reactors (as some ponders will do). My flow-through rate for each UV is approximately 1500 USG, so together at this rate, they turn over all the water in the pond once every 4 hours. They are rated for up to 3000 USG each, but I've never tried running that volume through them, so I can't tell you how effective they'd be running at high volume. They are very effective operating at 1500 USG each. The UVs have thin rings around the housing to allow the light to shine through, so you can easily tell whether or not the bulb is working. They don't come with a mounting bracket of any sort, but I use steel pipe strapping. I did order the UV from this supplier, and the unit arrived quite quickly. I was very pleased with the service. My unit did not come with "¾-inch, barbed intake and outlet for attaching flexible tubing" as stated in the listing, but rather with 2 x 90 elbows for 1-1/2" tubing. I intended to use my own straight adapters, anyway, since I use 1-1/2 inch flex PVC, so this was not an issue for me. Each ponder has his/her own preference when it comes to UV clarifiers, but I've had success with this unit, and I have not had any of the troublesome issues some others here have noted. Given the wide range of very high cost UV units out there, these fit my budget and function quite well. The replacement lamp and sleeve are reasonably priced, too, and if you're shopping for UV clarifiers, be sure to check the cost of replacement lamps and sleeves: some of them are very pricey, and if you want them to function effectively, as a minimum you should replace the bulb after 12 months of continuous operation. I replace mine seasonally and have kept a few of the "used" ones as spares. If I decide to add another UV, I will not hesitate to buy this one again.