Come on - give us a bit of a clue here.
what sort of tank - open, with a roof, pressurized??
Size?
Flow in / out?
Storing - water? sulphuric acid??
A static header tank or one which has large level changes
What controls its level?
What do you think?
Any overflows, pressure reliefs?
Now you might understand why there is no mysterious "code" to tell you given the huge number of variables.
A "tank" can be up to the max overflow level - generally 95% of the nominal capacity, down to 30% for a pressurised tank used to accommodate expansion.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.