Somewhat to my surprise, my dictionary does not include the word 'cubical'. In addition, Google gives over 22 million hits for 'cubic', and only 0.5 million for 'cubical'. So, I'd say 'a cubic' tank.
My dictionary OED does contain "cubical". But it suggests that the word is used only in special mathematical terms like "cubical parabola". Webster Rosette says it ranks as the 106,044-th most frequently used word in English. That is, it is used once per a billion of words people speak or write.
it is used once per a billion of words people speak or write. Wow, it's been used 7 times so far in the text of this thread. Now, every time we use 'cubical' (now, that's 8 times), I feel like we're wreaking havoc with Webster Rosette.
I particularly like the way this claims to be Earth's largest dictionary. It kind of opens the door to the possibility that Mars may have a larger one.