Don’t let water get inside your scuba tank!
Moisture inside the tank is bad news for both aluminum and steel cylinders. The most common cause is an improperly maintained air compressor, so make sure your local dive shop is taking care of their compressor. Additionally it’s a good idea to never empty your tank completely, as an open valve on an empty tank allows any moisture in the air to get inside.
The problem with my friend’s tank is pretty obvious: the inside of a steel tank should be smooth and ‘silver’ looking, not rough and rusty. A lot of moisture inside the tank and the resulting corrosion is what caused his tank to fail.