All you need to do, to brace is go and buy a few pieces of acylic...get the thickness that the actual aquarium is...if the tank is 1/2" acrylic, then get pieces that are 1/2" thick.
Say that crack is 2" long...get two pieces of acrylic that are 4" x 4".
Sandwich the crack inbetween the two pieces of acrylic and use Weld-On #4 to bond the pieces to the tank itself.
Weld-On is what they use to build the tanks. It's like an acrylic super glue that actually melts the acrylic together.