With water, gravel and decorations your tank could easily weigh 12 pounds or more per gallon in size. That's 240 pounds for a 20 gallon tank. You will need a stable base for your aquarium. You should consider a stand made especially for an aquarium. Furniture is not always made to hold all this weight.
Silicone is used to seal between the glass pieces and the frame.
The silicone seal not only prevents leaks, but also cushions one piece of glass against the other, which aids in preventing pressure cracks. Glass aquariums are warranted against pressure cracks and leaks. Glass aquariums are very easy to clean, they don't scratch as easily as acrylic, and they are generally less expensive than acrylic. They're made in a wide variety of shapes and sizes - from 5 gallon up to 240 gallons and larger. Traditionally, glass aquariums came only in square or rectangles.