Abstract: A viewing window structure for an aquarium tank and the like includes a removable water pressure resistive viewing panel which presents a substantially uniform water-tight face to the water side thereof. The window structure comprises supporting seat means embedded in a tank wall adapted to receive the water encountering viewing panel. A flange of the supporting seat means is operative to sealably confront the water side of the window panel and to provide a substantially flush water-tight seal. Pressure retaining frame means and supporting hardware are provided on the dry side of the window panel.