I would work on that mandarin and getting him to eat prepared foods, specially if he looks thin. It;s unfortunate, but usually once they start to thin out its too late.
I started with small frozen food like cyclopeeze and then moved to baby brine and finally she eats everything I put in the tank. The same with my scooter. Took about a month of training for each but it worked, at least for now.
As mentioned above, they can go through a bottle of pods
a week if not quicker. Do you have a refugium? Thats a good place to put pods
so they can grow and populate the system