Spa Stain

I wasn't really sure as to where I should post this question, so I hope this is alright.

We have an acrylic spillover spa (see picture). It is white, which is bound to show anything and everything. However, from day one of having the spa installed it has had a brownish ring around the spa from the surface of the water. We would clean it, and almost instantaneously it would come back. It isn't very visible when the water level is high, but when you drop the level in order to get in it, the stain becomes very obvious.

We were told to clean it with just baking soda. Baking soda works okay, but it still is very hard to get it off. Is the water just dirty, or what is going on? Like I said, this has been going on since it (along with the pool) was installed (almost 3 years ago) By the way, our pool generates chlorine through salt.

http://i7.photobucket.com/albums/y29...05/Pool051.jpg

It's sort of hard to see the stain in this picture, but it's there!
http://i7.photobucket.com/albums/y29...05/Pool053.jpg

Thanks.

PB07, looks and sounds like oils (sweat, suntan lotion, etc), having bathers shower/ rinse off before using the pool may help. If baking soda works, it's not a metal stain, you might pick up a small container of soda ash (a/k/a pH up) and use that the same way you've been using the baking soda (it's a stronger base and may make the removal go a little easier) Be sure to keep your eye on your pH and alk when you use either method. If you can't do the soda ash thing, for whatever reason, let me know and we'll work out another plan of attack for you.
Best wishes - Waste :)