This might be dumb but ... I've heard that steel tanks are preferred because they stay negatively bouyant from full to empty and aluminum changes from negative to positive. A Catilina 80 CF aluminum goes from -1.8 to +4 while a Worthington 80 CF steel goes from -9 to -3. In each case the difference is (about) 6 pounds more bouyant when empty. What is the difference? Physics & common sense tell me that 6 pounds is 6 pounds. What am I missing?