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.
125g Reef Yellow,Tomini & Blue Tangs,Watchman Goby, Squareback Anthias,
60g Rimless Reef- B&W clowns,citron goby, mandarin, scooter,Barnacle blenny,firefish
125g Malawi Cichlid
90GFOWLR Red & sixline wrasses,Flame Hawk,Squirrelfish,Engineer Goby,Coral Hogfish