The lights for the tank has been on all day and K measured the carbon in the air and found it decreasing how is carbon moving between the air and the living things in the tank

Carbon is being absorbed by the photosynthetic bacteria and algae present in the tank.

The living organisms have a constant exchange of oxygen and carbon dioxide from the environment into their body. The carbon dioxide is released during respiration where as its absorbed during photosynthesis performed by plants and some photosynthetic bacteria. Similarly oxygen is released by these organisms during photosynthesis and absorbed during respiration.

Photosynthesis requires light for its occurrence. So as the light inside the tank was on, so the photosynthetic bacteria and algae that were present inside the tank were able to perform photosynthesis and they continuously absorbed carbon dioxide for that process.

So the concentration of carbon dioxide got reduced with time.

thinking it would be d not too sure but i think it's right