I have had smaller sea shells like 10 in the tank and no differences in the ph, as far as I can tell because the fish are really happy. The Shell is right under the filter witch is a 15-20 gallon filter in there and I don't know if that will make a difference becuase the shell has been there for about 4 days and no odd behaviors even from the betta. My betta is madd at me because I moved the rocks where the plants are where he used to hide or rest so know he hides behind the shell. :P
And I read the thread that you gave me and I did see something I did witch was to poor vinegar on the shell to see if it fizzed and it didn't so I am good.