freshwater fish and sea shells

Question
What is the effect of sea shells being in a tank with freshwater fish?  We have a Betta at work in a 1 gallon aerated  tank and they just placed a huge shell in the tank with him.  He has taken a serious decline in health and struggling for his life.  Could the sea shell have anything to do with it?

Answer
Yes, the seashell, putting it bluntly, will kill him.  The shell is releasing deadly chemicals in to the tank.  I would suggest that you take the shell out IMMEDIATELY and do a 100% water change. When you add the water back, be sure to use a water conditioner, a tiny bit of aquarium salt (for healing) and some stress coat (these will aid in healing hopefully).

You cannot put anything from the "ocean" in to fresh water tanks, as it changes the chemical makeup of the water.  Please research this online as there is plenty of info on this type of thing.

Good luck, I hope this answer has not come too late.