Is the mandarin eating? Since you have a small tank you might have success with weaning him onto frozen food. Try this: turn off all the flow in the tank, take a turkey baster and suck up some frozen food and then target feed the mandarin. I generally shoot mysis and bloodworms and brine shrimp near mine onto the sand bed or some rocks and she will come around and eat it. I started with smaller frozen foods like baby brine, cyclopeeze and plankton.
A mandarin needs to be fed several times a day, much more than every 4-5 days. They will hunt and eat all day long if there is enough food.
Good luck!
