We have had a few instances lately where customers have come back having cracked the bottoms of their aquariums. It is very important to keep in mind that to keep your tank from cracking you must have a perfectly level surface that is free of any debris or imperfections. A great way to compensate for imperfections in the surface that your tank is sitting on is to use our aquariums mat's which help protect your tank from variance in the surface of your stand.
We cannot stress this enough, having been a professional designer and installer of large scale aquariums I can tell you the long term and short term stress this puts on your tank is critical and goes for both glass and acrylic. If you do not provide an optimal surface for your aquarium the stress from the pressure and weight of the aquarium and water can cause the material to crack or joints to seperate. And from experience...manufacturers never warranty if you do not provide the right surface to place your aquarium on.
I remember taking care of a 500 gallon saltwater fish only tank for the MLB pitcher Barry Zito, one day the whole face of his acrylic tank cracked horizontaly across the length of the aquarium dumping 250 gallons of water on and down through his floor. When an inspection had been done they found that the original company who installed it 5 years earlier had not provided a completely level base for the tank to sit on and this caused the acrylic to be under constant pressure eventually leading to it reaching it's breaking point. So please make sure you do this, and try to always use our mat's as well as making sure the surface your tank sits on is as flat and level as it can be.