Never put the covering glass directly on the aquarium - leave small gap for air circulation and to fish feeding. We use only the thin and light glass, plexiglass (the thicker the glass is the more light absorbs, but the larger the aquarium is then the thicker the glass must be - the alternative is the use of two smaller glass elements). If we will use plexiglass then it will let the ultraviolet to get throughout to the aquarium. Ultraviolet has germicidal action. Each type of cover should be kept clean from dust and other dirt. If we grow surface plants than we should place the glass under small angle - then water collected on the glass do not fall directly on the plants. You need to properly insulate all electrical components attached to the cover which may damage because of the moisture.