I know I've mentioned this before, but here is some visual proof of both my problem, and my solution.
I don't know if this has any relation to the age of our pool, and its pale blue liner, but this discolouration has only happened in the past three years. The pool was installed in 2005; making the liner 14 years old (but still in very good condition).
The picture, above, shows how, occasionally, the liner becomes discoloured beneath the water line; it becomes a dirty dull beige colour, and looks quite unpleasant. It reminds me of the ceilings in old Pubs that had become stained by tobacco smoke; not unpleasant in its own way, but not a good look for a pool.
The solution is very simple. I top up the water to above the stained level, then empty a whole Kilo of powdered Vitamin C into the path of the three water jets, and wait for about 30 mins; then, hey presto, the liner returns to its sparkling original colour. It really is magical.
The powder isn't cheap; around €20 per kilo through Amazon. But what a difference it makes to a stained liner. It's now sparkling again.
I like to keep a couple of Kilos in stock; just in case.