what should I add?

Like I mentioned in your other thread, I think a Tang here would probably be a great addition. Captive red yellow Tang pricing has come down to where they are almost affordable again. Even on sale occasionally. There are also several different types of fox face that do a great job of algae control. If you do go the Tang route, make sure you pick carefully. Certain tangs like a powder blue for example should be the absolute last fish that you put in the tank because they can be highly aggressive otherwise. If you're thinking about that post here and ask the question again as to whether or not it might be compatible. There's lots of pros here