Coral and seashells are calcium carbonate based rock, like V3X said, and they do dissolove in water, can raise your pH and can raise your hardness levels as well.
For some fish, this is fine. African cichlids and goldfish do come from naturally hard-water areas, so can handle the high hardness and pH levels. Some fish, like many of your South American species, come from soft-water, low pH regions of the world, so adding carbonate based rock to their tanks can prove to be deadly.
Because people don't usually know the different regions or water chemistires their fish come from, most aquarist giving advice to beginners say absolutely no carbonate based rocks in community tanks (aka sea-shells, corals, tufa stone etc). It saves the beginner a whole boatload of hassel from dead fish and unbalanced water chemistries.
So the only fish I ever advise carbonate rock for are African Cichlids and marine fish (for whom ocean water chemistries are buffered by carbonate stone). I don't even recommend it be in the tank with goldfish. But if you know what you're doing regarding the water chemistry requirements/manipulations of your fish and tank, then by all means, use whatever decor you wish.
You can wash it as described by Matt Nace.
~~Colesea
