Can I use store bought white coral. I live next to the beach and there are tons of stores that sell bright white coral. I want to put in my freshwater tank but didn't know if the have harmful chemicals.

No it's actually good for your fish if you have some that like a higher ph since that's what coral does. Rinse it all off real good first maybe even boil it. Google coral in freshwater. Sures makes the tank brighten up. Happy shopping.