The quick answer is that provided the shells are clean, and that any once-living occupants have long gone, they will be fine as decorations in a freshwater aquarium, and even potentially a good thing if your tank water is very soft. I would soak them in very hot water for a while just to be sure they are clean, but any marine bacteria present should be of no concern. Depending on the trends in the acidity/alkalinity of your tank water, the shells will dissolve over time, more quickly or more slowly, depending on the pH. So, as already advised, if you value the shells for their present appearance, keep them out of the tank!
If you are interested, here is a bit more on the matter. If your aquarium water is soft, and hence poorly or un-buffered, (that is, low in carbonates) then sea shells and coral can be extremely helpful in releasing carbonates (and calcium) which will help to buffer water and prevent rapid drops in pH - thereby actually helping to stabilize the pH. How strongly this will occur will depend on the volume of tank water, the hardness/softness/pH of the tank water, and the amount of shells. A few shells would have very limited impact in a large tank. The shells would dissolve more rapidly as pH drops, which is precisely what you would want if you need to buffer the pH upwards.
If you are using a bio-filter and give minimal water changes, and especially if the aquarium water is soft and un-buffered, the pH can be expected to drop naturally. In this situation I purposely incorporate shell grit, oyster or abalone shell, or coral rubble into (homemade) bio-filters to buffer the water. This is a wonderful, safe and natural tool for doing so.
If you have fish which prefer acid water, and you were to use a lot of shells or coral, you would need to give more water changes if necessary, to ensure the water does not get too hard or alkaline for them, but for the average home aquarium this is unlikely to be an issue. It is nevertheless always good to monitor basic water chemistry, especially until trends are understood.
