Ceramic Stain: Removal (from hot water tank)
Sounds like a rust stain. I would go with a product like CLR.

But, before trying it, try it on a spare uninstalled tile to be sure it won't do more damage to the tile than the stain.

These products are very caustic, so don't underestimate them! Heed the instructions carefully regarding sitting time, use of personal safety equipment, and thoroughly rinsing the tile afterward.