I just built a 42g tank with no problems (it isnt leaking yet!!!

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Obviously make sure you use the appropriate thickness glass for the job and use aquarium silicon, also make sure to use the appropriate braces to strengthen the tank.
Also make sure that the glass is cut accuratly this is important as an 1/8 of inch out here and there wil cause you problems, so make sure the matching panes of glass are the exact same size.
If you can get somebody else to help you hold the panes in position as you assemble them this will also help, and i would imagine essential for a tank your size. Also use corner clamps and duct tape to hold the panes as you progress.
Spend a lot of time in preperation before you start, useing tape about 5 to 10 milimetres from the edge of each pane of glass, run the tape up each side to be sealed, when you have sealed with the silicon, remove the tape and this gives you a nice clean and straight edge, spend a lot of time doing this preperation, you will be glad you did when you finish the tank.
Let the silicon cure for about 4 days before you put water in the tank, and do a leak test for a few hours in your back garden before you put the tank in its final position.
I made my tank with no real problems and wouldnt hesitate to make a bigger one, just take your time particularily in preparation and you will be fine.
Any questions just ask
