Do you keep a log of your test results? If so, make a photocopy and send it to them to support your warranty claim.

If not, it's a good idea to start one. Record each day's test results. I keep a regular notebook (paper! pen! Luddite!) with date/time, pH, FC, CC, TA, CYA, temp. I don't always test all of those but when I do, I write it down. I also have a column for what I added, whether I vacuumed, backwashed/rinsed, whether it rained heavily, that sort of thing. Observations/actions/notes.

It's easy for them to say your cover got wrecked because of bad water chemistry. Make them eat their words and replace your cover by proving them wrong.