I could not find the leak in the summer so I have reinstalled the tank and all looked good, until... winter. Found a trace of diesel in the aft water tank compartment as the water tank was removed for inspection. Diesel tank goes out, another pressure test and checks ... nothing. Then before installing again decided to leave it for a few days on the boat in the position where forward bottom port corner was the lowest. After a few days - there it was, diesel seeping from this corner. Decided to reinforce it as this corner takes beating when boat is tossed around on waves. Will see if my fix pays off. I do not want to remove/reinstall the tank again as it is a major PITA.