I've been using industrial type silicone for my tank and so far it did not pose any harm to my tank. Actually I used the silicone before knowing that it may cos harm to the fishes. Lucky nothing bad happened.....

RayTan,
I normally be back home in the evening and at that time I would like to sit down and enjoy looking at my tank. Thus I've set the light timing from 2pm to 12 midnite. If you're doing that to your tank then the lighting will be in excess. Although the windows are closed the sunlight will still be hitting the tank. Better if you have a curtain.