behavior

Is there any particular reason people keep the little rubber caps on their tanks? Is this just a convenience for dive shops as a way to indicate whether a tank is full or not that just spills over to divers who own their own tanks? Especially for a yolk valve, I don't see much harm in letting it...

Why do juvenile sweetlips move in such an odd way? I don't think it's anti-predatory, since they don't change location much during their dance. The continuous jerking movement surely must use a lot of energy! Even more curious, the adult fish are very placid. Has anyone done research on this...