I have just started to look after a neighbours liner pool with the plastic steps. I have noticed on the steps that there are dark stains appearing randomley on the steps only. Where each stain appears, it has raised the platic and seems to give the impression that it has split the plastic.
There are no metal elements to the pool ie pools steps, and the pool is a normal liquid chlorinated pool. Chlorine is at 2.8 ph is 7.5, and the current pool temp is 20c.
I have added a picture to see if anyone has seen this before.