Pondless Rocks have turned black

[PondSpirit]

I have had this pondless waterfall for 11 years, and have been able to keep the water and bed rocks clean with 1 cup of hydrogen peroxide about every 10 days. Six months ago, I had the pump, the liner thoroughly cleared of debris, by just emptying everything, and even wiping the pump reservoir clean. Worked beautifully for 5 months by continuing to add hydrogen peroxide. But this past month the bed rocks look like they have become smudged black in spite of the hydrogen peroxide, and the black cannot even be rubbed off by hand. Water looks clear, no algae on rocks or floating.
Nothing else in the garden has changed. Gardener suggested bleach, but the rocks didn’t clean back to their original state. I then turned off the pump. After several days, the rocks look “white” again. However when I turn the pump back on, they quickly turn black!! I am attaching 2 “blacked rock” photos. Thank you for your help!!

 

[Lisak1]

It does look like bio-film to me. How many gallons is this feature? I'd try more hydrogen peroxide - it's cheap and can't hurt anything. I'd opt for that over bleach as you may have birds and other creatures that drink from your water.

 

[addy1]

I would leave them black looks natural and adds filtration to your pond

 

[Stephen Noble]

Hi PondSpirit, welcome to the forum! Has the waterfall has been operational for 11 years with the same rocks? Were the rocks originally 'coated' with something to keep them clean or shiny? If so, the coating is almost certainly gone after this much time and use. If I read this properly, the rocks now have a dark color that can't be rubbed off. You mention that bleach does not return them to their original state. What does it do to them? When you rubbed them prior to the bleach soak what color did they turn? What color are they if you let them air dry? What percentage of hydrogen peroxide (H2O2) are you using? Sorry for so many questions but I find your post interesting.