putting some things from outside into the aquarium?

[baldegale]

i have a couple rocks and big freshwater clam shells that id like to put in. theyre all from other waterways such as lake erie and rivers around me. ive already been putting in small (prob about 1/2in) shells and it hasnt been doing anything. i just wanna know because the shells kinda big. ill try and post a picture of the rock and other shells if need be