First off Round-Up breaks down fairly quickly. It was one of the selling points about “killing” an area and being able to reseed after ten days. I would still give the rock a good wash. I wouldn’t use muriatic acid though…that’s too strong an acid and will cause for some of the rock to dissolve. Go with vinegar if you want to do an acid wash, but not too long, again, because it will start to dissolve the rock.
I would do multiple soak/rinses in tap water. Put it in a bucket and let it sit for a day; dump out the water and repeat. I would do this for or five times.
Now you’ll need to cure the rock in saltwater, so into a tank/tub/Brute with a heater and powerhead for a month or so. Monitor, at a minimum, nitrates. It they get high, do a water change. Continue to cure the rock….and you’re good to go.
