Collecting rocks or wood locally always has risks, some of which can be eliminated while others remain unknown risks. Rock like wood will soak up liquids along with anything dissolved in that liquid. There is no way to leech these out rapidly, so they may slowly leech out over months in an aquarium.
I would not advise using rock collected under salt water, which is what I am assuming you mean by finding these on the "beach."
I have used river rock that I buy at a local landscape supply. This is granite, so not a problem as calcareous rock would be dissolving minerals and raising GH/pH. It is worn smooth by river currents, hence the name, so it is generally safe. This is very inexpensive; I bought two buckets of various sized river rock for 75 cents a bucket, so for $1.50 I aquascaped my riverscape tank. Being "river rock" it is also very natural looking in an aquarium, and thee varying sizes allows you to make quite interesting aquascapes.
I would not advise using rock collected under salt water, which is what I am assuming you mean by finding these on the "beach."
I have used river rock that I buy at a local landscape supply. This is granite, so not a problem as calcareous rock would be dissolving minerals and raising GH/pH. It is worn smooth by river currents, hence the name, so it is generally safe. This is very inexpensive; I bought two buckets of various sized river rock for 75 cents a bucket, so for $1.50 I aquascaped my riverscape tank. Being "river rock" it is also very natural looking in an aquarium, and thee varying sizes allows you to make quite interesting aquascapes.
