Up to 24" height is typically ok with .5" thick acrylic.
If it's 30" tall, it would probably bow some if the acrylic is only .5" thick. At 30" tall I would want it to be 3/4" thick. I have seen quite a few 30" tall tanks that were made out of .5" thick acrylic and they all bowed so much that I passed on them.
Tank dimensions are typically listed as L X W X H. If it's 24" tall, the seams look ok and it's listed at a reasonable price, I would go for it.