This big fish may be an image of yourself. You try to kill the big fish because the big fish (that is, yourself) has an attribute you don't like (such as being too strong, too developed, or too greedy, eating the small fish before dying). For you, because the bad habit of the big fish is your own weakness in your subconscious, you have a fear in your dream and the idea of "trying to make it die of lack of oxygen". You want to kill yourself.