I know RobbieH is a TFP expert, but I'd never use a metal brush on glazed tiles. I used a pumice stone on glazed tiles and it ruined them.
I've used Comet and a rag, SoftScrub and a rag, tile cleaner from the pool store, plastic scrubbies with some kind of cleanser, straight & diluted muriatic acid w/ a nonmetallic brush, & CLR with a rag. Sometimes, it seems, one technique works better than another. I've used as much as a whole can of Comet, a bottle of SoftScrub or tile cleaner, a quart or so of acid and never had it affect my water for more than a few minutes. Even then, it was only the water near the cleaning, not the whole pool.
Maybe I've been lucky, but this my experience on the same pool over 20 years.
Good chore for kids, if you've got any. Good luck!