Starfish in a freshwater fish tank?

One of the girls that works for me in my kennel brought me some shells she got in Florida. There is a conch and another conch type, two HUGE starfish and a fairly large sand dollar. They were purchased, so have been sterilized and are supposedly safe for putting in a fish tank as they haven't been treated or anything like that.
Just so happened that when she gave them to me I was setting up the new 40 gallon I got today. I did put in the conch and the other one like it (it actually looks like some kind of sea creature with long legs but is a real, hard conch type shell). I am, however, concerned about putting the starfish AND the sand dollar in the tank. I am worried not only about destroying them but also about contaminating my tank.
They are beautiful and HUGE and I think they would look really good in this tank with lots of plants as well.
Anyone have any positive OR negative thoughts about using them in a tank???

They would quickly harden your water, adding minerals. That would be fine for livebearers or East African cichlids, but major environmental stress for most tetras.

I wondered about that, especially since I already have really hard water. Okay, display case for them!!!!