Hi Blepore;

You don't want to acid wash a pool yourself. It's to be avoided anyhow, since it works by ripping off the top layer of your pool's finish, leaving it nice and white, but also rough, porous, and more easily stained.

Go to Walmart or where ever and get a small container of Iron Out (detergent / stain section). Make a slurry of it, wet a stained area of your pool (no standing water) brush or sponge the slurry over as much of the stain as you can, and see if it lifts it. If it does, see how much area you can cover, and then estimate how many times that area you need to clean.

You can get a larger quantity (65 oz) from Amazon:
Super Iron Out IO65N Rust Stain Remover
but if you are going to need a LOT, call Summit Brands and ask them for a delivered price on a 50lb container. They will sell direct.

One caution: if you have asthma, do NOT do this. Iron Out contains sodium hydrosulfite which can trigger severe acute attacks in asthmatics.

But, using Iron Out will NOT damage your pool, and will not leave you with 1,000's of gallons of acid and water to dispose of. It's not damaging to small pumps, so you can hook up a little sump in the pool, with a long hose, and run the water into the nearest toilet and the sewer. (It won't hurt sewers, but it's not good for grass!)