There is no "May have handled it better" about it Steven, they absolutely could and should have handled it better. Sending a letter weeks after the fact starting it with "you hold in your possession misplaced goods" is an awful way of trying to resolve the matter.
It would have been far better to call in person (or at the very least over the phone) to explain their mistake, apologise for it and then discuss how it could be resolved. To compound the matter by then being rude on the phone just worsens an already bad situation.
Also you say it's easy to measure the tank, however you missed the point that the mistaken delivery was 5 weeks prior to the threatening letter so any measurement would be 5 weeks late.
The company need to back down here, apologise for their bad handling of the situation and then discuss how it can be resolved (and they really should be willing to do better than a measly €30 discount!).
Until then the OP is under no obligation to take the next step imho.