Using an oil tank as a cistern: Who has done that?

Aha. From your statement that the things must be deregistered when decommissioned, I concluded that they must then be registered and subject to regular inspections like heating appliances - otherwise, deregistration would seem pointless to me.

To my knowledge, this is also definitely the case with underground tanks. Personally, I removed the oil heating system along with the tanks in the last house. There was a building permit for the containment tray, and the tank was also registered with (I believe) the municipality. It is not inspected in Lower Saxony. In other federal states, it is, as far as I know. In any case, there is now somewhere in a file that my tank was disposed of by the specialist company.

We only became aware of the whole issue because the saleswoman forwarded us a letter from the water authority stating that a proof of tank inspection is still outstanding. The tank is therefore registered. Fortunately, they were very nice there and one phone call later it is sufficient if we submit the decommissioning certificate in a few months. For this, we received a list of SVs.