Def. 1/4 and it is better to use Acrylic. You can use Plexiglas but in a year or so the Plexiglas will star to turn yellow and bend. And also you do not need the "special" Fish Tank silicone, if you go to home depot and get the GE "silicon 1' it is fine to use, I actually called All glass and asked them and they said that it is the same as their silicone.
CHris C