Doing a complete drain can destroy a liner, so it is not usually a good idea to do so. Also, there is no reason to change out sand unless you add something that gunks it up like doing a conversion from Baquacil to chlorine, etc. I am on the 12th season with my same sand. I have topped it off a couple of times but that is it. Take Ben's advice and get the Taylor K-2006 kit. You need to be able to test for more than chlorine and pH and the good kit will test for everything you need.

Good luck.