Black stains on pebble tec pool

[Paul1231]

I stated using a robot pool cleaner a couple of months ago. Around the same time I dropped a pool rx device into the filter basket. This is the first time I've used either device in three years of owning the pool. Since then I've noticed these black streaks on the walls of the pool. I think they're from the rubber tread on the pool cleaner as it climbs the walls. Based on the photos I've seen I don't think it's algae, but I could be wrong. I've tried cleaning them with a nylon brush and a stainless steel brush. The stains won't budge. What can I use to remove these stains?

 

[Texas Splash]

What can I use to remove these stains?

As a test, do you have a chlorine tablet laying around? Maybe rub a stain with one of those for a short time? Don't leave the puck in one spot, just move slowly around to see if chlorine will do anything.

 

[ajw22]

PoolRX adds silver, copper, and zinc to your pool. Your streaks may be metal stains. We never recommend systems that add metals to the pool for that reason.

 

[bowenpan]

you might have black algae hidden in between pebble tec stones. grab a toothpick/sharp object and dig out some to confirm.