It is not that hard to build a tank, I have built several. It is very true that it may not save you much money. The thickness of the glass is very important. I have recommended before that you copy the size and thickness of a tank you see in a pet store, and look at Oceanic brand tanks, as they don't use tempered glass, which can be thinner with the same strength. It is more expensive to buy though. Use silicon that is 100 percent silicon, and make sure the package says "safe for food contact." The most important trait of the silicon would be that it NOT have a mildew inhibitor in it. When I have called to get numbers on thick glass for a large tank, it was not worth it to buy the glass compared with just buying the tank. Of course there are no limits to the size of a tank you build yourself!!!
There is also no guarantee that it won't break, and guess who is to blame???
Sonny