Clownfish

Clownfish are food items that are obtained by fishing.

Fishing
The most common way of obtaining clownfish, like any other fish, is fishing. They are rare, as the chance of a catch is 2%. The chance of one being caught can be increased with the 'Lure' enchantment.

Guardians
Another method of obtaining them is through defeating Guardians. This method is very inefficient, as the drop rate is roughly 0.05% (2.5 * 0.02).

Food
One clownfish restores hunger and 0.2 hunger saturation.

Taming mobs
Clownfish can be used to tame ocelots, like most other fish.

Trivia

 * Clownfish could be considered one of the most useless items in Minecraft, as they have no use that normal fish/salmon do not. This combined with the fact that they are quite rare to catch makes them very inefficient to use. They also cannot be cooked, making them a poor choice of food (They only restore half a hunger bar currently).