Taken from the BDSG website

The carriage of SCUBA cylinders (or any breathing apparatus cylinders) by private individuals for personal use is exempt from the provisions of the Carriage of Dangerous Goods legislation provided that measures have been taken to prevent any leakage of contents in normal conditions of carriage.

If cylinders are to being carried by someone who is at work, there is a requirement to carry at least one 2kg dry powder fire extinguisher in the vehicle. The rest of the regulations do not apply as long as the combined capacity of the cylinders is 1000 litres or less (which would allow the carriage of up to 83 twelve litre cylinders).

Read the full document HERE