Pond Shield non-toxic epoxy can be used to create faux rock and other ornaments for your aquarium. Basically you start with a piece of high density foam. Regular white foam can be used but it is not the best choice because it tends to fall apart too easily. High density foam works better because it can be cut cleaner and even sanded, if needed.
You can use tools like the foam cutter we use from Hot Wire Foam Factory. They have a variety of foam cutting and carving tools that make creating foam pieces much easier that other methods.
You can use spray foam to glue the foam pieces together. Once the spray foam cures, it can be cut just like the high density foam and shaped any way you wish. Be sure to wear gloves and protective breathing devices as the cutting tools will be very hot and the foam fumes are bad for you.