It is a bad idea to put any trigger in a 20gal tank or any reef tank. At the risk of sounding harsh, you really need to select appropriate fish for your size of tank. The impressive tangs, triggers and angels are out. Look into smaller fish like the sixline wrasse, roual gramma, various gobies or purple firefish. You are limited on space but do have some very nice options that will work well in your 20gal tank.
Some people are like slinkies...they serve no real purpose yet can still bring a smile to your face when you push them down the stairs!:p
Have a great day! Brian