first of all....15 watts is not much UV for 1500 gallons. A lot depends on your system though. sun? shade? Heavy fish load? Type of filtration? Also, UV's have a stated flow rate. If you push more water thru them than the stated flow rate, they will not work. It may be that the gph you are pushing thru the light may be too much. What is the flow rate the UV suggests? You can always install a couple of Tees in your like and divert only part of the water thru the UV.

ccoombs1 is right on the dot. Improper flow and wattage can make a UV not work at full capability. It could be something as simple as the bulb not working though. Did you handle the bulb? Oils from the skin can destroy UV bulbs. Can you tell if it is even on? A lot of times the simplest thing can be overlooked. We've had a lot of problems with transformers on the larger UVs, does it have one? Just a few possibilities.

I run a 18 watt on my pond and all stays clear with full sun. You need to make sure the flow is slowing going though. The slower the better..

What size wattage should I be using? The light is on, you can see the glow, and there's no transformer. I slowed down the flow rate, it cleared somewhat, but not enough.

I'm looking at my chart and I have one company's 10W @1500gallons and 700gph, and another at 9W @ 1800gallons and 900 gph. Looks like most 18W run at about 2000gph max, so you're somewhere in the middle.
The wattage is plenty so thats shouldn't be an issue, trying to figure out how much gph is the next goal I would say. Most UVs I install I just hook up to a seperate danner mag-drive pump. If you have to mess with splicing into an existing line and valving to the right speed it can get a bit tricky. I'd suggest going out and buying a 1200gph Danner MagDrive pump and just using that. Drop the pump in the pond or skimmer and have a hose emptying into the pond somewhere you can disguise it a little. If you can splice into the existing line and make sure you're around 1000-1250 gph, thats great. Also, UVs act quickly, but if you've slowed it down and its helping I'd say give it a couple more days and it could be fine as is.

The flow rates mentioned seem too fast to me. I have a 25 watt UV for my 1000 gallon pond. I connect a 500 gph pump to the UV, but the flow is reduced by the head getting up to the unit. The real flow is probably around 350 gph. My system will turn pea soup to crystal clear in 2-3 days.

I would try to reduce the flow down to 200-250 gph. Slower flow increases the exposure to the UV and will eliminate the algae faster.

A 15 watt bulb seems low for a 1500 gallon pond. Reducing the flow should help, but the low wattage of the bulb is going to increase the amount of time required.

I currently have a 15watt UV light for my 1500 gallon pond, and it doesnt fully clarify the water. Im actually looking for a bigger light now becuase when the weather is cool the pond is clear, but as soon as its sunny/hot green goes the water.

Everyone has different success and failure stories. I wish there was a one size fits all, but we all know that isn't true. I have some charts from a couple of ladies in lawrence, ks, that are some of the best in the nation. UVs can be a bit tricky to get right in the sweet spot, but the numbers I've quoted are either from professionals or from the manufacturers themselves. Feel free to try more wattage or less flow if it doesn't seem to be enough, that would work 'more', whether thats better or not depends on your individual results. Good luck to everyone.

I am doing a series of adjustments on my new 150watt UV that is supposed to clear my 11,250 gallon pond. The UV is rated at 19,000 gallons but to date (will be 2 weeks on Saturday) it has only cleared the water slightly. I am not giving up on it because it comes from a manufacturer we all know and trust.
I will keep you posted. Here are some photos. The first one is the way it looks today and the second one is how it looked when I first started up the UV. I have been emailing a rep at the company on a daily basis providing them with update photos and they are saying in the end I will have a crystal clear pond. I sure hope so for the price of this thing.
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Here is a link that might be useful: My pond the way it was before the freeze.

bugsb, What a beautiful pond! How long did that take you to create? What is the average depth? How long has it been up and running?

Thanks for the kind words. We got it up and running in August of 2005. It took several months of work and down time waiting for friends to add their skills but in the end I am pleased with the results. The pond is 30 inches deep. My only problem has been the green water and I thought this time I would get it for sure with a $1000.00 clarifier. Hmmm I hope I haven't waisted that money. It doesn't grow on papyrus.

Here is a link that might be useful: My pond the way it was before the freeze.

Thats a decent sized setup you got, what are the mfg reps saying? "Large pond = long wait to clear?" "Need more watts or flow?" I'm curious what they're suggesting. Honestly I thought a UV that size would have cost a good chunk more. Good luck with clearing it up.

Thanks for the follow-up! Several month of time well spent.. Keep us posted as to your UV progress...

They have had me tweak the ball valves on the manifold I built for it. Their idea is to slow down the flow from my 6000GPH pump. As you can see I have now closed the return valve from the UV partially to cause a back pressure. That I am estimating will slow the water going through the UV to about 3000GPH. They say slowing it down will cause the water to be in front of the light system longer and will hopefully kill the algae. It should be noted that in their literature this UV can be run up to 6350GPH. Maybe that is just in a maintenance mode and not in the original kill. At least that is a guess. I will let everyone know if this works.
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Here is a link that might be useful: My pond the way it was before the freeze.

Just wondering if anyone ever uses an actual flow gauge when trying to restrict flow to the UV clarifier?

I just use a separate fountain pump for my little UV light. I still have to use a diverter. It works but the bulbs are hard to find. I bought a new larger one last year on sale for $35 but haven't had time to install it. The fish like to play in the discharge. Sandy