Tanks should be visually inspected once a year according to industry guidelines and to manufacturer recommendations. A visual inspection checks for signs of corrosion, structural damage and contaminants that can affect the air the diver breathes. This should be performed when advised to avoid any safety issues in this potentially hazardous sport.
It is Not a Requirement
There is no law in place, unlike hydrostatic testing, that requires the tank to be periodically inspected. Visual inspections cost money and with no requirement the money saved could be spent elsewhere. A diver performs a routine check of the tank exterior before every dive anyway, so this could be considered sufficient.
Safety of scuba divers is paramount and therefore all precautions that can be taken, should be. Tanks can pick up contaminants from bad air fills and corrode inside, even if well cared for. A visual inspection checks for these and therefore should be done annually, according to recommendations.