I'm sure anything from the pool store will work. This is Keetrol Plus Stain & Scale remover. It says it cures metallic stains, rust and scaling in pools. It is to correct mineral problems in pools. Toilet bowl cleaners are acid based, this stuff is not.
I am afriad of simple green since it reacts with aluminum. I like "oil eater" cuts grime in the bilge pretty well. I use simple green in the cockpit and tonight to wash the Aqua-buff off my deck.