Also in my 57 I have a lot of rock. I also currently have 12 fish. Got a few for free for a guy giving them away so I ended up with more than I wanted or should. But my tangs go and sleep in the rock every night. You say give them plenty of open space. But don't they need plenty of rock to sleep in also. Or hide in when scared. My hippo darts in and out of the rock all day. Might be that the tank is too small for it so not acting normal
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I have rock piles with lots of caves and hiding places, but there is also a lot of open space and straight line runs so the fish have room to swim, yet have protected places too.