Hammer loving life

Is it normal for clown to stay at the top like that? Parameters are normal.

Some what beginner here: If my nitrates reach 20 ppm. Should i just feed a bit less to get them down? Would these levels be dangerous to corals/fish?

Why would some of my Zoa Polyps not be opening. These pictures are 2 weeks apart.

AB+

Honest opinion, whats it lacking? My Clown fish always hanging around in the back corner. Its a 15 gal tank. How many fish can i put in it?