Tank bows out when full....please help

[Aquattro]

You know, I've never seen a 33g with a centre brace, and 4mm is really not much. That's with 300 pounds of water in it, so if it's only bowig 4mil, I wouldn't worry about it. Now if the tank is old and has been dry for a long time, that might be something to worry about. Silicone can become brittle with older tanks that have sat empty for long time.