Have you ever seen a car tire blow out? If you're anywhere nearby, it will definitely get your attention. Consider for a moment that a car tire is usually inflated to about 30 psi. Now, multiply that force by 100 and replace the rubber with metal and you've got some idea of the job a scuba cylinder does.Air cylinders - tanks - are probably the most overlooked piece of equipment divers use, yet they are key to recreational scuba diving. Also under-appreciated is the fact that your tank can have a profound impact on your dive. Whether you're looking to buy or just renting on vacation, knowing a little more about them can significantly reduce strain, improve your bottom time and increase your fun.You Do Have a Choice. Most recreational divers are accustomed to seeing and using 80-cubic-foot (cf) capacity aluminum tanks at resorts. They are the most commonly used tanks in the world. This popularity, however, has as much to do with cost-effectiveness as with performance. Depending on the diver and the dive mission, the aluminum 80 may not be the best tank for the job.For individuals who are outside of that wide generalization called ''average sized,'' there are many tanks that may be better choices than the 80. Smaller divers, especially those who are good on air consumption, may find that a 50- or 68-cf tank is a better fit. A smaller tank won't hit the backs of the thighs during finning and will be a lighter load to bear while getting in and out of the water.Larger divers may find an 86- or even a 120-cf tank more to their liking as it's not uncommon for larger people to use more air, and a big-capacity tank will help them extend their bottom time.Positions People, Please. Positioning the tank on your BCD is a simple process that is frequently overlooked in the rush to get into the water. But a tank that's mounted too high or too low can prevent you from achieving a good attitude - a stable body position with your head up slightly from horizontal - in the water. By following a few easy steps, you can substantially increase your diving comfort.During your pre-dive safety check, tip your head back so you're looking up at the sky. If the back of your head is touching the regulator's first stage, your tank is too high.A high tank puts you at risk of banging your head during a giant stride entry. (Remember: Your head should not be tilted down toward the water.) It will also limit your ability to keep your face up and your eyes on what's ahead while you're in the water. A low tank is also a problem, and if it's trying to grip on the ''shoulder'' of the tank, it is more likely to slip out of your BC tank strap than one that's properly positioned.In addition, a low tank can affect your hose positions. It can pull on your regulator second-stage hose, causing jaw fatigue and neck strain. It can also put your power inflator hose in an awkward position up on top of your shoulder or cause it to flip behind your back.Again, check this during your pre-dive routine. With your regulator in your mouth, rotate your head left to right a few times. The hose should not be pulling the second stage to the right, and you should be able to move comfortably. (Also check that it's not pushing the regulator too far to the left. If so, the tank may be too high or the hose may be too long.)If you can't find a compromise position between the first stage hitting your head and the low tank pulling your hoses down your back, there is a simple solution: longer hoses. There's no rule that says that you need to keep the original hoses that came with your regulator. Again, they were designed for the ''average'' person. That doesn't mean they'll fit your body type. Your local dive shop will no doubt carry a variety of hoses to help you solve your problem.Once you've found the proper position for the tank on your BC, make a visual note of its position and write it down in your logbook after the dive. Jot down the tank size, and use easy measuring units to describe the positioning - for example, ''top of the valve stem is two finger widths down from BC collar'' or ''distance from bottom of tank valve to top of BC tank band is one hand length.'' Use whatever system works for you. Many BCs are now fitted with a tank adjustment strap that is typically an adjustable loop of webbing attached above the tank cinch strap. The adjustment strap loops over the neck of the tank to ensure the BC rides at the same height when placed on a new tank. Remember that any measurements and adjustments will change if you use a different size tank.Of course, if you own your tanks, all of this only need be done once. You can then mark the position of the BC strap on the tank with a permanent marker or duct tape or by scratching a line.Buying or renting, the key here is to ask for what you need. Most dive operators and dive shops carry more than just aluminum 80s and are very willing to accommodate special requests.SELECTING YOUR TANKSTEEL VERSUS ALUMINIUMSTEEL: Many steel tanks are negatively buoyant, even when empty. This can be a big advantage for larger divers who normally wear a lot of lead on their weight belt. On average, a steel tank will allow them to remove about 3-6 pounds of lead. Even though this is largely a case of ''where you put it,'' it can make things more comfortable.ALUMINIUM: Aluminum tanks are generally negative when full but become positively buoyant when the internal air pressure gets down to about 2,000-2,200 psi. Unfortunately, the aluminum 80 is the most positively buoyant tank when emptied to nominal pressure (300-500 psi).Remember, however, that you will gain about 5-7 pounds of positive lift by the end of the dive, no matter what kind of tank you use. This is one of the reasons we're taught to test for neutral buoyancy when there is 500 psi remaining in the tank. If you are neutral with 500 psi, you will be slightly negative at the start of the dive. You can compensate for this with your BCD. As you use air, you will become less negative. You won't, however, go past neutral to the point of being positively buoyant.If you find yourself struggling with your buoyancy or haven't been in the water in a while, PADI's Peak Performance Buoyancy Specialty course is a fun and easy way to perfect your skills in this area. John Kinsella is PADI America's director of training and quality management.