A fish tank illumination system including an external light unit placed against a transparent panel on a fish tank filled with water, and a decorative element submerged and placed inside the fish tank. The external light unit includes a light source producing a narrow light beam through the transparent panel. The decorative element or adapter is mounted on the inside surface of the transparent panel opposite the external light unit. The decorative element may include transparent or translucent objects or internal cavities illuminated by the light from the external light unit. The adapter may be used to receive the light from the external light unit or used to mount the decorative element on the transparent panel. Magnetic elements in the external light unit and on the decorative elements are used to align and hold the light unit and decorative element or the adapter on opposite sides of the transparent panel.
