Actual shells in a tank?

[CaptainMako]

I found a box of sea shells, types and sizes varying from tiny to bigger than your hand, in a box in my garage today. These were most like gathered from the ocean off Pensacola, Florida many years ago.
I don't have a tank yet, but I thought it would be cool to put some in there, along with large sections of dead coral that I also found in the box.
I know these need to be cleaned profusely, but do they still pose some kind of threat to the health of my tank if placed inside even after cleaning? The coral I am doubting to put in there, just under the assumption it it may break apart or crack if placed in water again.
Is this a terrible idea?

 

[Lowryder]

Coral and seashells will raise your ph. So if you are planning a fish that likes a higher ph, by high I mean around 8, you will be fine. Africans will enjoy the coral and shells.

 

[JohnPaul]

The issue with adding coral, or seashells, is that they will leach calcium carbonate into your water. This will make the water harder and also raise the pH.

Is that a bad thing? Not necessarily. Maybe yes, maybe no. It depends upon what you plan on putting in your tank, and also upon the chemistry of your tap water. Many people who have softer/more acidic water will actually keep a bag of crushed coral in their filter in order to increase the pH & hardness of their water a little bit. And in pet stores you can actually buy aquarium substrate that is 100% crushed coral. However, this would not be used in your typical/normal general freshwater tank, only in something where a high pH is wanted, like a high-pH African cichlid tank.

So the answer is basically: "it depends." Depends upon what fish/creatures you want to keep, what the chemistry of your tap water is, and how much of that stuff you are considering using (relative to the size of your tank). One or two seashells is probably not a big deal no matter what. But if you are talking about putting a couple large chunks of coral into a 10g or 20g aquarium, well, now some of those other questions become much more relevant.