FYI sharing this link regarding some laser optics and laser maintenance information.
http://www.parallax-tech.com/faq.htm
There is always discussion on these forums about cooling water.
From this source:
If you are using city water, filter it.
If you are using a closed circuit flow system, check the water and change it every six weeks or so, or drop a few drops of Algaecide in the recirculator. Better yet, use diluted Dow Frost.