Hi there. did i see right that your nitrates are zero? Also, yes the tang is supposed to be in a 6ft or larger tank, they actually need ALOT of swimming room. I just got my 2nd SW
setuo, a 55g and trust me I would LOVE a tang. part of the reason I got another tank. But unfortunately I dont' feel right in putting him in a four foot long tank, it wouldnt be right for the fish.
Please bear in mind the fish store will tell you what you want to hear to make a sale. Think how much you paid for the tang....fish store guy doesnt care if it gets stunted or catches a disease he got your $$ already. I've also read where a tang is more susceptible to varied diseases when kept in small enclosures. Just some food for thought.
With your current stocking i'm not sure you can really get anymore fish, maybe a mate for your clown but you seem pretty packed full already.
As far as the string coming from the leather, mine currently is doing the same thing. long sringy thing blowing with the water flow. He looks very healthy so I figured I'd give it a few days.