Stain on bottom of the pool

[gordoben]

Hello,

I have a 3 year old 40,000 litre pebblerete salt water pool.
The pool has been troublesome free since installation.

We have had trouble getting salt lately so have been using liquid chlorine.
Finally when i got come i put in 2 bags and the pool went green.
I noticed a patch of very light brown stain on the pool floor right in the middle.
Put in the robot and gave it a manual scrub to no avail.
I spoke to the pool shop whom said wait till winter, only few months away here in Australia as when treating it less likely to go off.
I spoke to another pool person yesterday whom suggested i bomb it with hydochloric acid, approx 10-15 litres, make it really acidic and scrub the area a few times and see if the high acid in the pool helps to remove it. I have never heard of this method before, have you? Would there be any downsides to this. Swimming season is over in our household so we wont be using it.

My biggest nerve is that this pool is only a few years old. We only use tap water to fill it up. I would hate this for staining to be permanent. Also, i have noticed that the chemicals used for stain removing can end up being fairly expensive.