As long as you do not have anything bad in your pond, then it should be fine. I have seen people take rock out of creeks, just have to be careful no chemicals are in the water source.
You could always use a container and pour water from a water change into the container and leave a light over it to grow algae. This will stop contamination.
You should be able to grow algae on the rocks in your tank. Put the rocks you want in there get a bulb for growing plants and let nature take its course. By bulb I don't mean you have to buy a special light just find probably an actinic bulb for your current light setup.