Answer:
The answer would probably be; No.
Most tropical fish like a lower pH than the ocean. Not all, but most.
Sea Shells are made of calcium and slowly dissolve in your tank. They will raise the pH.
They might not even look as if they are giving off anything but they can.
Unless you have fish who like a high pH I would not add them. You can also test to see if they cause much changes in fresh water. Put some distilled water in a small bucket. After it stays exposed to the fresh air a day with de-chlorinator in it. (lets the pH swing from the air happen.) Test the pH in the bucket and record on something. Place the shells in bucket. Test the water every day for a week and record. The odds are the pH will slowly drift up. If the shells are soft it will raise fast.
You can use this test on rocks you also wish to place in your tank. I once had a pH drift and thought it was my decorations. They soaked in a bucket for a few days and the pH did not ever change. It turned out my gravel was a bad batch and the manufacture sent me replacement bags! I would of never thought the gravel was contaminated until I soaked some in water outside the tank. The pH kept going up!! It had small pieces of sea shell in it!
This is where I say the opposite:
Growing up I kept Cowry shells in my Tropical fish tank. They are very hard and dense. The ones I had did not seem to affect the water quality much. They are some of the harder shells one can get. Water changes are also a must. Most tanks require at least 20% a month using a hydro-vac. If you like Neon tetras and other fish that like a pH of 6.5 to 7. I would not put any shells of any kind in your tank.
Hope this helps.
