Adding seashells to your freshwater aquarium can be beneficial, but they should not be the only decoration in the tank. Seashells provide a natural look and feel to any aquarium, as well as offering hiding spots for fish. They also create small crevices that are perfect for beneficial bacteria growth which helps keep water quality balanced. However, it is important to note that some shells may contain high levels of salt or other minerals that could affect the health of your fish if added directly into the tank. To avoid this problem, you should always rinse off any shells before placing them in your aquarium and make sure they are free from dirt or debris. Additionally, you should use caution when selecting shells - many types of mollusks (including clams) have sharp edges which could injure delicate fish like bettas or goldfish if placed in an overcrowded environment. Finally, never add broken pieces of shell into your tank as these fragments can cause damage to filter systems and other equipment inside your aquarium. With proper care and consideration for safety measures taken when introducing seashells into a freshwater aquarium, adding these elements can help enhance its beauty while creating interesting hiding places for aquatic life within it!