Periodically scraping the back glass on your aquarium may be time consuming, but it does have some benefit other than aesthetic. Light bounces off the glass and can illuminate the backside or your reef. This may not seem important to you, as the light reflected isn’t all that much, but all parts of a coral need light to some extent. This will help keep the full colony colorful and healthy, not just the top and front of the coral. If algae and other forms of life have accumulated on the back glass, they will absorb the light. Likewise, paint on the back of the aquarium will also absorb light.