Nice Mandarin.
In my 180 with about 300 pounds of live rock, I couldn't keep a Mandarin alive with enough food.
After studying the situation for awhile, I was pretty convinced that the several cleaner shrimp I had in the tank at the time were probably completing with it for the same foods.
I have sinced cut back to 1 cleaner shrimp in that tank. I'm hoping that the next time I add a Mandarin, I'll have better luck.
Based on my experience, a 20 gallon with competing cleaner shrimp may be an uphill battle for your Mandarin over the long term.
Since the tank is small, it will be easy for you to keep and eye on him and monitor his condition.
Best of luck,
Roy