Thanks for all the great information. I am a pool service technician and have found a lot of useful information here. I have kind of an odd question I thought I'd try. I just took home 20 water samples from some testing. I loaded them up in clean, empty water bottles. That was 8 days ago. They have been stored in my house where it is cool, with the caps firmly closed, and a towel laying over them so they were not in direct sunlight. Any ideas if these samples might have changed? I am planning to test for PH, TA, CYA, CH. I did some testing to see if CH might change over several weeks with some old samples and they tested the same, just not sure how/if other tests might be effected. Thanks for the help.