ok ive got an idea which is srsly cost effective and will add a heap of strength to ur tank
when using ur base use floor railing. the idea is to doble or triple the surface area on the edges. so what u do is (if u can talk mm) get 2x 10mm thick rails(long glass peices that are 1800mm long and 50mm wide.. glue them together.on on top of the other wo u get essentially a 20mm peice of glass that isnt a s strong as a 20mm single peice of glass. then glue the 50mm side to the edges of the tank so that all edges are covered but the peices do not hang over the edge so they sit flush with the edge. so what u get is 20mm+the thickness of tyour base. which is possibly doubling if not tripling the surface area. i believe sticking to a 6x6x27" tank can work with 12mm glass remeber lots of top bracing and lotsa glue. i tmay also help to get some alluminium peices that are 4mm bent 90 degrees and adding glue to that so it helps resist the seams popping out. btw building anything larger then 6x6x27" in 12mm is a serious gamble and even that size may not work. ive also noticed that two faces are not viewed. back thees up against a vertical 2x4 frame so as to stop any bowing(which is why glass snaps) if u have any uestions or cannot understand the floor railing concpet pm me