Operating water depth is most important in choosing the right compressor- The oxygen transfer efficiency is dependent on the contact time the air bubble has with water. The deeper it is the longer the contact time and better diffusion of air and oxygen in water With change in depth the surface area changes, this means for a smaller depth there is larger surface area and lesser oxygen transfer efficiency. So there will be need for larger compressor (higher flow rate) and also more number of diffuser to cover area and also to diffuse more air. Volume in all case will remain the same; it is the depth and surface area that determines the compressor choice.