It should clear the pond in 5 to 7 days. It can be a sudden change, meaning it may not get clearer on days 2,3,4.

There is suppose to be an indicator light to tell you if the unit is operating.

If it doesn't clear it doesn't mean the bulb isn't strong, it may mean the flow thru the unit is too fast, or too slow.

I don't find any indicator light. If it's working it should clear as last year I had no problems with algae.

The density of the algae determines the flow rate. Thicker the algae the less the light penetrates the water so the slower the water has to flow past the light. So it varies year to year.

You can turn down the water flow by say a 1/3 and see if that clears the pond in 5-7 days. Once clear you can turn the flow back up.

You can also remove the water and replace. Another algae bloom will no doubt happen but slow enough for the UV to keep up so you never see the bloom.

UVs are meant to keep a clear pond clear. They have to be way over sized to handle an already very green pond at max flow rate. The cost of the bigger unit and all the electric needed is lot compared to just changing water once. But many people do buy bigger units. Global warming is our kids' problem, not ours after all.

Whatever you do, do NOT look at a lit unfiltered uv bulb. It will damage your eyes although you may not realize it for some time.

I did find the indicator light so it's working. Thanks!!