i think your problem may also be trim. sounds like you are tilting underwater as well.
issues at the surface have been covered, so i'll stick to underwater:
get into a horizontal (on your tummy) position under water and clasp your hands.
breathe normally, and see what happens.
one of four things will probably happen:
your head will dip: this means you have too much weight forward of your center of gravity or cg)
your feet will dip: this means you have too much weight aft of your cg
you will roll to the left: too much weight on your left
you will roll to the right: too much weight on your right
also, you could experience a combo effect (such as rolling left and aft, or right and fore).
to eliminate left and right roll, reposition heavy items that may be dragging you in that direction, including evenly distributing your weight left and right. also, position items
as close to your cg as possible.
to eliminate up and down roll, you will need to re-distriubute weight along your
central axis. move the tank down or up, shift weights from your waist to BC pockets,
or to strap pockets, etc.
what you want to do is (1) identify which way you are rolling, and (2) move stuff
around, including your tank, to achieve flat trim.
hope this helps