Zelda Link's Awakening: missing key

In the angler's dungeon, there is one room where you need to get a key to open a door in the lower levels, but the key chest does not appear.
To make the chest appear, you have to kill all the enemies in the room, but it never appears.

Solution 1:

It seems your picture is not from Angler's Dungeon (number 4) but the Bottle Grotto (number 2).

If I recall correctly, this room is a puzzle in which you have to kill the enemies in a certain order.

A tablet somewhere in the dungeon could help you find out the order.

TABLET / CLUE SPOILER :

First, defeat the imprisoned Pols Voice,
Last, Stalfos...

Answer : The order is

Bunny, Bat, Blue Skelteon