Posted by: Mike Clark
Found this after a quick Google.
A BP document on the storage of diesel. Copper and zinc both cause accelerated ageing of diesel resulting in gummy deposits. Any water in diesel can cause growth of bacteria which form slimes which block filters - I would guess that this would be less likely in a copper tank as the copper is a strong biocide as was well known to HM navy in the days of wooden ships. Comment was also made that the slime problem is more severe in the case of biodiesels which have added ethanol which causes water to separate out and encourage the bugs.
Is there any chance that Jim is always buying from the same fuel source which may be selling diesel already contaminated with water! One suggestion as to the source of the copper carbonate is that water in the tank is absorbing carbon dioxide from the air, producing carbonic acid which reacts with the copper. Water may condense in the air space of the tank - always keep the tank well filled or drain it completely over winter storage.