The present invention provides a liquid tank, a viewing device for under-liquid observation, and an optical film, each excellent in visibility and durability and being easy to clean. The present invention is a liquid tank having a transparent wall, including, on an internal surface of the wall, a first moth-eye layer having a moth-eye structure, and a protective layer covering the moth-eye structure, in this order from the wall side, and preferably, further including, on an external surface of the wall, a second moth-eye layer having a moth-eye structure, and the second moth-eye layer being disposed in a region facing the first moth-eye layer.
