[fieldnstream]

When I did my 55 I used 3 cans of GS, 4 tubes of silicone, and only 1 diffuser panel. I only did the back though, if you are doing the sides as well you might want to get 4 cans of GS, it goes way more quickly than you would expect. One thing that I did that made the tank look a lot better was to put black contact paper on the back and sides.....might be worth looking into.
Good Luck!

 

[fieldnstream]

You only need 1 panel....use 2.5 or 3" pvc connectors to raise the sheet of diffuser. Contact paper is self-adhesive paper....its used on the outside of the tank....not necessary but looks much cleaner than seeing silicone and GS.