I'm pretty sure your tile is bulletproof. It's most likely porcelain and will resist just about anything that would try to stain or etch it. Acid (stronger than vinegar....say about 1:5 muriatic) SHOULD be fine on the tile. Actually, done with care, it should be fine on the grout as well.....just apply it in a small spot so if you see it harming the grout you can stop. (It'll have a tendency to lighten your grout a little but, again, done carefully, it is an excellent scale remover.
If you apply the acid/water mixture and it doesn't foam, it's not strong enough. If it foams violently for a minuite or so....Oops! too much. Throw some pool water on it.
First, I would suggest cleaning that line with something just a little soapy (Ajax or comet in small amounts would be fine) to remove any organic scum. I would then suggest mixing 1:5 (muriatic to water) to start and, in the pool with rubber gloves, begin to apply the solution to the scale line. I think you'll be impressed with the results.
Like anything else, a big dose of common sense goes a long way when handling acid.
Well, the Italian dressing analogy has whetted my appetite. Time for the house (my house) salad! :lol: :lol: