How full should an aquarium be?

An aquarium should generally be filled to about 1 inch below the top to allow for water movement and to prevent overflow when adding decorations or when fish swim near the surface. This ensures adequate oxygen exchange and reduces the risk of water splashing out. Additionally, leaving some space helps maintain a stable environment for the fish and plants inside. It's essential to follow specific guidelines for the type of aquatic life you have, as some species may require more or less water depth.