Soak, scrub with vinegar, rinse and rinse again. All decor, tank, etc.
Are they old live rock? If so, they'll be leeching calcium carbonate, etc. which may or may not be desirable for your freshwater fish. Fine for an African setup once cured out, but it will take a couple of weeks for all the dead stuff to rot out if it was live rock. Just let it soak in water and rot, or get a filter going and let the decomposition cycle the tank for you before adding any fish.
There are a lot of questions and possibilities in your question. Please tell us more about the rocks and the tank. Appearance of rocks, anything that comes to mind.