@MikeHolm:
i think Mad Fridgie referred to the length, and i gave the figure i was handed by the installer. you are right of course regarding the Chromagem tank spec. and regarding the postulates of physics.
instead of feeding you with partial/incorrect/irrelevant data, i contacted the tank supplier for details, and hopefully, he will send me some kind of a spec sheet with actual drawings. since most of the suppliers here religiously believe in the "buy & go (to hell)" principle, i can only hope for a miracle (isn't the holy land a fine place to do so ? :-).