Welcome to the Planted Tank Forums. Glad you've joined us.
I have zero experience building tanks, but with that being said, I've been reading up on it, intending to make my own at some point. There are a few good threads on the forums, use the search feature and read them. I don't have the bookmarks to give you the links, so you'll have to do that yourself.
There's a few key things to consider.
1) What is holding things together? If it's just silicone it had better be a smaller sized tank, and you'd best find the best quality silicone available, as some of the DIY threads on here seem to indicate a stronger hold with industrial grade stuff.
2) Do it once, and do it right. When you put everything together with silicone, it's put together, there's no going back. It is pretty near impossible to clean off 100% of silicone from the glass after it's applied, and without a clean surface, a new layer of silicone won't hold as well. In other words... if you put it together once, and it leaks, putting it together again may just yield even worse results, as the new silicone is now going to be bonding to old silicone remnants, not to the clean glass surface.
3) Tempered glass resists breakage better than non-tempered. But which you choose is a bit less important than the thickness. But ultimately you need to ask WHY you are building this tank. If you are trying to save money on something that will hold many gallons of water in your home, you are playing with fire... er... water. When you compare price differences between tempered and non-tempered, and 1/4" vs 3/8" always remind yourself: What I'm building could potentially cost me hundreds or even thousands of dollars in repairs if I don't do it right.
4) If I'm trying to save money, I'm probably going to lose unless I:
A.) Already own most of the things needed
B.) Am avoiding paying a custom fabrication company to build this for me, and I know what I'm doing.
C.) Don't value my time. (Even if you consider your time at national minimum wage levels, how many dollars of your time have you spent just researching?)
D.) Don't need things to look super fancy. If you look around online, some people build plywood tanks. They aren't necessarily cheaper, but can be more sturdy, and if you already have plywood leftover from some other projects...
Christian, husband, brother, friend, hobbyist.