Make sure you know your tank. When I was in Guam the dive shops used C80's which are a better al tank the the S80 that you find everywhere. The C80 remains negative the entire dive whereas the S80 becomes Posative. In reality that does not matter as it is the weight of the air in the tank that you lose throughout the dive. You should MAKE SURE you are negative at the end of the dive regardless of ttank you use. Remember with an 80 (actually only 76cubic feet of air) the tanke will lose approx 5 lbs over a dive. So you may be negitive at the start of a dive BUT you may very well be posative at the end.
Pete
Pete