mm wrote:

Stronger, I think, than vinegar but safer, I think, than muriatic acid
would be Zep Acidic Toilet Bowl Cleaner sold at Lowes. They sell a lot
of Zep products in their cleaning and janitorial section. It has
hypochloric acid, which sounds similar to muriatic (hydrochloric) but
is probably weaker and whose warnings are less. And it's already
diluted. And it's also a bit thick so it clings to the sides better
than muriatic acid would (which I have somewhere and have used a
little bit.) Blue with yellow lable on white plastic bottle.

Muriatic Acid = Hydrochloric Acid