Should I aerate year round?
As water cools it is able to hold more oxygen thus lowering the need for aeration. That being said many people do choose to keep at their system running to keep an opening in the ice. This can allow toxic gasses to escape and prevent winter fish kills. If you live in a region that gets extended 0°F its best to not run the diffusers in the deepest part of your pond to prevent supercooling.
How should I place my diffusers?
Diffuser placement is about finding a balance between pond depth and distance from the other diffusers and the shore line. You should always have one diffuser in the deepest region of the pond. In a pond with a consistent depth it’s best to split up the pond evenly with diffusers. In a pond with an uneven bottom it’s better to have the diffusers closer together to target the deepest part of the pond.