Abstract: A tank or other form of container or vessel for holding fluids being treated or the like is provided with a removable cover. The cover is formed of a number of panels of a composite material including a thermoset synthetic resin and a reinforcing fiber material typically fiberglass. The panels are formed by a pultrusion process and are of sufficient length to extend between the end walls of the tank as single units. The panels may be made of any suitable width and depth and require no welding or joining to form a single panel width.