Corals look great!!
But that tang has GOT to come out soon. They for sure cannot live in a 20g tank. They stress very easy and it will eventually kill him. :-( Not to be the bearer of bad news. Sorry.
That tang should be in a minimum of a 125g 6 foot long tank. Some people squeeze them into 75's which is better than a 20g, but still not big enough in my opinion.