Glassfin Minnow

Source
Glassfin Minnow can be fished from the waters of Crystalsong Forest in both the river and from schools and also in the chambers of in Ulduar.

As an ingredient
Minnows can be used by an alchemist to make Ethereal Oil:

The oils can be used to make Elixir of Water Walking, which is a discoverable alchemy recipe:

Achievements
Fishing from a Glassfin Minnow School is a criterion to complete.