I have used Jack's purple initially and see it disappear over a course of 2 weeks with running a higher water level. We keep it from coming back by using a test kit for sequesters (available from Jack's) and using a weekly maintenance amount. Works great for salt pools and tile as well. You may need to up your doses depending on your hardness levels. Lew Akins in Texas also told me of a set up that is used on household water as well. It uses a magnet I do believe.
Tell me a little about your fill water and pool water balance. Get a water analysis of these and find out where your alkalinity is and the calcium hardness is. Other items I'd like to know is: do you use a calcium based chlorine in your pool? what type of filter do your have? There are chemicals that can help reduce this ring but first we should find out the basic stuff.
I will assume that your water chemistry is in a good state. That said, one must remember that when water evaporates, only hydrogen and oxygen ions leave, theis leaves minerals attaching to the tile line. The higher the mineral content of the water the more rapid the buildup. If you keep the water level in the pool at a consistent level the scale line is less noticeable. Adding smaller amounts of water more frequently helps in this as does the addition of an auto fill device.
Your local home depot has a product called Sulfamic Acid. It is a small tub of crystals. Mix it with one gallon of the hottest water you can and apply it to the calcium on the tile either with a brush & bucket, or use an acid sprayer. Agitate with a brush, then rinse thoroughly. This type of acid is even safe for glass tile. Just be sure to rinse really well. If you get it early, before the calcium has a chance to mix with oils & other substances floating on the pool surface, it's easier to remove. You can even do this process weekly until the calcium is under control. It is also possible that the calcium is leaching out from the grout, not just from hard water. It's best to be careful with seqestering agents. They contain phosphates, which is an agae superfood, especially in the presence of a salt chlorine generator.