If people see that you are taking care of them they will show their appreciation by returning time and time again. Charge more for your services. Organizations who concern themselves with safety will pay more for complete, diligent inspections. In the interest of promoting scuba tank safety, this page was formatted as a courtesy to the Advanced Inspection Technology company and was based largely on one of their brochures. Their Visual Plus product has been recommended to me by several people as being a breakthrough in scuba tank safety.
Some cracks can be so small that they are essentially invisible to the naked eye.
The Visual Plus device uses electronics to catch hidden defects that are difficult, if not impossible, to find otherwise. If, for whatever reason, you cannot discard or refuse to fill these tanks, consider requiring the tank to be electronically inspected with an eddy current device at least twice a year before you fill them.