How to remove stain on waterfall rocks

[brucen]

I made a big mistake. I had an algae problem in my small pond and waterfall. I bought a heavy duty algicide containing copper. This was for a large area of water. So, I had to dilute it. But I miscalculated and added 5X the correct amount. My pond immediately turned blue. I immediately drained the pond but the rocks on my waterfall were stained blue (but not the concrete on the bottom and sides of the pond, only the boulders on the waterfall)l. It has been over a year and the blue stain on the boulders remain. Is there anyway to remove this stain. Or, can I spray paint the boulders to cover the blue stain. Thank you! Bruce

 

[GBBUDD]

One way to do this is with a blow torch. But it depends greatly on the rock. If you use even a plumbers torch and SAFETY GOGGLES not glasses as the rock should start to exploded but only on the surface. Again depends on the type of rock. I had cobble stone that was painted as curbes brite yellow and grey . All gone.
Muratic acid/ hydrocloric acid is another possibility and so long as its diluted of it makes its way to the pond you should be fine. Providing you dont 5x