ADR
Tank size is important, does not matter how much you carry. For instance if the 2000 litre tank is empty you are not allowed to tow it without an ADR licence unless the tank has a certificate of cleaning to say it is empty ! So basically no ADR licence you can only tow 999 litres of Jet A1 or 333 litres of Avgas
The licence lasts 5 years before you have to do a renewal