Yellow Head Jawfish (Opistognathus aurifrons)

Jawfish are burrowers, and this one takes it further than most — it excavates and maintains its own burrow in the sand, shoring up the walls with bits of shell and rubble the way a mason picks stones, and it will spend most of its life hovering just above that burrow, ready to duck backward into it at the first sign of trouble. That vertical, tail-down hovering posture is genuinely unlike how most fish swim, and it's part of what makes jawfish such an endearing oddball in the trade. They need several inches of fine sand to actually build a proper burrow, plus rubble mixed in to reinforce it, so a bare-bottom or shallow-sand tank isn't going to let this fish do what it's wired to do.