Marina 4-inch Long Air Stone – A straight 4" aquarium air stone (sometimes called an air diffuser bar). When attached to an air pump, it creates a line of bubbles that rise up in the water, improving oxygenation and adding movement. Because it’s a longer bar shape, it can be buried under gravel to produce a wall of bubbles or placed along the back of the tank for an even bubble distribution. It’s excellent for aerating larger areas in the aquarium, and the constant stream of fine bubbles helps release CO₂ buildup and keep water healthy for fish.