You indicate you just upgraded your UV to 25W. So I'm assuming the whole nine yards is new.. I.e. bulb, housing, transformer, etc.. If you keep Koi I'd say you are at the minimum of the wattage needed. I have found suggested wattages ranging from 10W/150 gallons to 8-10W/1000 gallons. From the dimensions you provide your pond has 300 cu ft. which is about 2250 gallons. At the lowest suggested range your UV is probably just barely the right size. (8-10W per/1000 X 2250 = 18-22.5 W). Using the highest recommendation, - 10W/150 gallons, it should be 150W. That's pretty high I think. I run a 3500 gallon koi pond with about the same fish load you have, and also a 16yr old pond and I use a 40W UV, (2 20W bulbs). I think I'm on the low side of what I should have for a UV and I'll be upgrading next summer. My UV struggled this year as well and I didn't have as clear as water as I usually do.
The other thing that a UV depends on is the flowrate past the bulbs. I have no idea what the flowrate of your filtration system is but if it is not in the range of what is recommended by the UV manufacturer, you are not getting the full 25W benefit. To high of a flowrate will reduce the effectiveness of the UV.
My GUESS is that your Koi have grown and are producing a heavier waste load on your filtration system providing more nutrients to the algae. This is not an uncommon situation as Koi mature. This year you might have hit a tipping point. I don't know where your location is but the US had a mild winter and if you are more south, your algae might have not have been suppressed much over the winter months and might have gotten off to a really good start where your UV just couldn't over come it. Obviously other factors such as plants, sun exposure etc can be a factor but since you've had the pond 16 yrs that shouldn't have changed much this year over years past. Any chance you fertilized your plant any extra this year? That could also contribute to the problem.
All IMHO! :razz:
Craig
