I am also in Texas with hard water but have "new" salt water pool for 10 months and have had small amount of ring-around-the-tile for which I used (only once so far) scrub brush and a bottle of retail cleaner called Comet recommended by builder. Worked fine.

What I also have is some scale build up in grout between tiles and in slight discolouration of grout I think from colour leaching out from gunite. For that I use pumice stone from home depot, works easy with 1 hour of elbow grease.