I agree with everyones previous posts.
Padi teaches to do a weight check prior to dive where you float at eye level with an empty BCD while holding a normal breath. Thats the proper way, but with a full tank of air its not totally accurate.
The compressed air in your tank has weight (normally about 5 lbs). As the dive progresses that weight is altered as the tank slowly empties. By the time you reach your safety stop with 500 lbs pressure, you are around 4-5lbs lighter than your pre-dive. So, if you were neutrally buoyant at the beginning of the dive, your now positively buoyant at the end because your 5lbs lighter.
1. Do a weight check with a full tank so that you float at eye level with and empty BCD holding a normal breath. Once achieved, add another 5lbs of weight (either on your belt, weight integrated, or tank weights). This added weight allows for the 5lb loss which occurs as your tank empties.
2. At the end of your dive, do another weight check. The goal is float at eye level with empty BCD holding normal breath with 500 lbs pressure in your tank. If you sink past eye level, take off a couple of pounds. If you float above eye level, add a couple of pounds. If you need to adjust in smaller increments you can always look at carrying something that weighs 1lb or 1/2lb. in your BC pockets.
You start the dive negative so you can end it neutral....or slightly negative.