Any glass tank will bow over time, even 12 inch thick glass. Glass is a supercooled liquid, it is a liquid. Look at the windows in a 30+ year old house, it takes a long time for gravity to cause glass to flow, but under the pressure of a filled aquarium it doesn't take more than a year or so. The amount of the bow is really insignificant and does not affect the structural integrity of the tank, the silicone sealant will fail long before the glass will. If, however, you can deflect the glass by pushing on it, the glass is not strong enough for the pressure of the water behind it and it will eventually burst. A tank seems to bow the most in the first year or two and eventually seems to reach an equilibrium with the pressure behind it for some reason. If you buy a tank from a reputable mfgr like All Glass or Perfecto, etc. there is no reason to worry about it.