This is the 8297th most frequent Russian word.
мелкое
"Мелкое" means "shallow" or "small."
'Мелкое' refers to the shallowness or low depth of water.
На этой реке место очень мелкое.
In this river, this place is very shallow.
'Мелким' describes the trivial and insignificant nature of the action.
Его поступок был мелким и эгоистичным.
His action was petty and selfish.
'Мелкую' characterizes the size of the fish as being small.