Tempered glass should be fine for an aquarium, as long as you don't try to cut it or put holes in it for plumbing. Although tempered glass is stronger than plate glass, once it's tempered, you're stuck with it as-is. It'll shatter if you try to cut it.
It's definitely cheaper to buy a pre-made tank, unless you can scrounge up free glass. I've come across so much glass it's unbelievable. People just throw the stuff away. I found sheet after sheet of 3/16" glass at the dump at my cabin. People redo their windows, and throw out the old stuff. I've made quite a few small tanks with it, although lots of the glass was in rough shape and unuseable. No leaks or explosions so far.
You could also try the buy and sell.
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Slr mclaren