An aquarium round air stone is a device used in fish tanks to increase oxygenation and improve water circulation. It typically connects to an air pump via tubing and releases fine bubbles into the water, enhancing the exchange of gases and providing a gentle flow. Air stones are essential for maintaining a healthy environment for fish and other aquatic creatures by ensuring adequate oxygen levels and helping to prevent stagnant areas in the tank. They come in various sizes and shapes, with the round version being a popular choice due to its even distribution of bubbles.