There are a few things to concider about adding seashells to an aquarium. Firstly they are a calcium carbonate base which means that in unbuffered water they will raise the pH to 7.6 (max). As someone that keeps and breeds african cichlids this is not something I concider to be high. I keep those tanks around 7.8 to 8.0.
While that is all said and good, most tap water does have natural buffering capacity that changes with location. This can be measured with a GH test kit. What this will tell you is how hard you water is and how likely it will be able to buffer changes in what is added into the tank. If the water is soft, then the seashells will have a greater impact than if the water is hard. This is because as the shell breaks down, which is will do, it will overcome the buffering capacity of your tank water if it is soft.
All in all, the most important thing for fish is consitancy. Most fish have an amazing ability to adapt to the water conditions of their locality. Which is why we are able to ship them all over the world without worrying about the water chemistry at their destinations. So even if adding the shell raises the pH a little bit, it shouldn't be enough to bother the fish, as long as it is able to be kept in those conditions long term.
So check your pH, the hardness of your water, and see how much of an affect one or two shells has long term.