Sooo...just to be clear...you were told by a 'pond guy' to cough up your hard-earned $ for a bulb that may or may not need replacing? Stop & think about that for a bit...

QUOTE: My water was crystal a month ago but is now getting murky.

Lots of experienced ponders here willing to share their knowledge but you're not likely to get any [accurate] reponses sans details. What's your water:plant:fish ratio? What type of filter do you have? And don't forget the pump. What zone are you in? Are you just starting up your pond or has it been in operation for a while? And last but not least, define *murky*. Is your water cloudy, brown, green?

In the meantime, hold on to your wallet.

Ditto on the murky thing. UV only fixes green.

There is equipment to test UV rays, but I've never heard of any pond store ever having one. They can be tested by looking at them for a long period, if you go blind the bulb is working.

You could shuffle the bulbs until a combination worked, but that doesn't sound like much fun.

An older bulb may have a black smug at the ends near the contacts. Don't look at a lit bulb.
I agree we need more info.

While the unit is plugged in, pull the intake or output hose off one end of the UV. Do not look directly at the bulb, but looking into the unit at an angle, you should be able to see a bluish or purplish glow. If it's dark, the bulb needs replacing.

Could you quantify "pretty expensive" as to the cost of the bulbs?

I use the 11w bulbs that mail order were around $20-25 each.

Went to Lowes, and they sell them for $10.95 all the time. I replace them in the spring every year, if for nothing else peace of mind.

And my water is looking good.

Coxy,

I would guess that you have a Savio standard skimmer. I am guessing that you have 2 50 watt UV's. The thing that I can not know is if you have Savio UV's or Emperor Aquatics UV's. If they are Emperor you may be able to find someone besides Al Zimmer to sell them cheaper. Al is the brother of the owner of Emperor and everyone watches that he is not selling Emperor products cheaper than MAP pricing. You may find someone else that won't advertise any cheaper than MAP but will sell them cheaper if you contact them.
UV bulbs by most manufacturers are to be changed when they drop down to 80% of their new output. Most of the time this is 9,000 hours of use or about 13 months of use. There is no way to tell by looking at the bulb if it is still operating at the level that is needed. The metal that is coating the inside of the bulb at the ends has nothing to due with the age of the bulbs. Whatever you do don't look directly at the bulb when it is on even for a short period of time. The UV will be burn your eyes very quickly and you will not know that it has happened until later.
Mike

Mike, I am using 2 Emperor Aquatics 50 w bulbs and to Groundbeef, they are $60+ each. So, it sounds like replacing them every year is the deal. That clears (sorry) things up considerably.