You mentioned that your tank was rolling on your back. How snug is the harness? Your trim may all be a function of a loose harness. As far as weight is concered, Only have the weight neccessay to maintain neutral with tank(s) at 500 psi. The other thing is move the straps for the tank up and down to help you find the center of gravity for you and the tank(s). The fact that you have a pool to work in is a great place to "fiddle" with the gear issues. What does your body do when static in the water column? If you go head down, slide tank back in the straps, if on the other hand you go feet down, move the tank up with straps near the crown of the neck. The only other thing is RELAX!!!!!, your breathing will slow down and your bouyancy will not be the yo-yo it sounds to be. If you have a friend who can take video or stills that will be extremely helpful in analyzing your positioning in the water column. Keep up the work, and compare what you have learned to what was toally unknown prior to the class. Keep up the work, your bottom times and gas consumption will increase proportionately. Your physical fitness will aid in this as well.