Never stop breathing or hold your breath, but do breath slowly, if you want to be slightly more positively buoyant to rise, then start inhaling before you have fully exhales. and vice versa to sink. With practice you can learn to breath with your lungs full or empty. My lungs have a capacity well in excess of 6 litres, probably about 8 to 9. When I am breathing underwater I only inhale and exhale a fraction of that, say 3 L, so I can use the rest of that capacity to adjust my buoyancy. Research into IPE ( immersion pulmonary edema) and immersion pulmonary oedema (IPO) strongly indicates that gentle slow breathing is advisable particularly at increasing depths and gas densities.