The image of Tobias and the fish is one of the central images in the book of Tobit. Interestingly, there are very contrasting interpretations of this image from St Augustine and St Bede.

St Augustine says that the fish is Christ. In the story, the fish leaps out of the water to eat Tobias' foot. The fish, St Augustine says, is offering itself to Tobias. The heart and liver are used later as an incense to drive away the demon Asmodeus (the king of demons) from Sarah. The gall is used as a balm to heal Tobit's blindness. This is the image of Christ's death driving away the demons from humanity and healing our spiritual blindness.

In contrast, St Bede says that the fish is the image of death and satan. Tobias, the son of the father (Tobit), is the Christ figure in the story. Satan tries to consume Christ but only gets his foot. Christ then destroys death. This interpretive paradox is amazing to me because we can take both at once to form a single image of Christ trampling down death by death.

As an aside: this is a great example of how Biblical texts can have various correct interpretations without being contradictory.