Sydney Cardinalfish

Found in small schools, during the day sheltering in rocky areas of estuaries, harbours and off-shore reefs, dispersing at night to feed.
They feed nocturnally on zooplankton
Southwest Pacific - Southeastern Australia

This fish is endemic to Sydney Harbour (hence the name) and Botany Bay.

Males incubate their eggs inside their mouths, as the egg masses increase in size they can be found with wide open mouths where they shift the eggs around to aerate the egg masses.