He was probably talking about cyanuric acid (CYA) and it is necessary in an outdoor chlorine pool to keep the sunlight from destroying the chlorine. I suspect that you are chlorinating with trichlor tabs which do containg cyanuric acid. Once it build up to proper levels your chlorine started to hold. However, this is not a free lunch! The CYA will continue to build in the pool and you will reach a point that you need to run your chlorine levels higher just to keep up. See this post.
http://www.poolforum.com/pf2/showthread.php?t=365
The problem you will run into with the trichlor is that as you add more chlorine you are adding even more CYA! This will eventually cause you problems!

This is why most of us on the forum use bleach for chlorination.

What this pool store told you was basically true!