Welcome to TFP!
Sounds like you've been through the wringer at the pool store, much like many people before you and myself a few years back. The good thing is that you found TFP and you want to do something different, that will work.
Getting yourself a good test kit is the first and most important step in taking control of your pool. Here are the TFP recommended test kits:
Test Kit Comparison I'd highly recommend the TF-100 from tftestkits.net as it is the best value and has all the reagents you'll need to take control of your pool.
Contrary to what the pool store has led you to believe, your sand is almost never the real problem. By the time it gets in your filter, it's already millions of years old. 5 years in your filter is nothing for "wear" on the sand. At worst, it may need a deep cleaning.
I'd recommend getting a test kit ordered, leaving the pool store behind (mixing their advice and ours is BAD) and get yourself acquainted with our methods in Pool School. I know you probably want a quick fix, which is what the pool store is always selling, but seldom delivering. Do yourself a favor and forget the big money quick fix and take over your own pool using TFP. Your wallet will thank you for your patience and finding TFP.
To help you better, please add the details of your pool to your forum signature. See how here:
To change your profile and/or signature, select "Settings" (top right under Pool School button). Scroll down (left side) to "My Settings" & select Edit Profile or Edit Signature.