Toilet Ring

I have a couple of inexpensive toilets...one in my art room and one in the shop. They don't stain. I have two expensive toilets in the house. The immediately developed a red line at the water level. I tried everything to get did of it but nothing worked . A woman told me to pour a two liter bottle of Dr. Pepper in and after a while, wipe it with a dryer sheet. Well, it worked! Sprite would have looked better, though!