From Halopedia, the Halo wiki
|This article is based on canon information, but the article's name is conjectural, and/or uses descriptions as the title, as there is currently no official name for the subject.|
The fish have a teal tints, with green spots on its body.
The Covenant apparently make similar use of them as humans do of their counterparts, namely that of a decorative pond fish. A school of fish was kept in one of the Valleys of Tears in High Charity.
These fish only appear in the Gravemind level, and can not be killed and are confined to the pool they are spawned in. The fish when hit by an explosion will be flung within the confines of the pool. When the fish get near one another they may start to follow one another, and when the player gets near the fish will attempt to evade the player.
In Halo 2, the fish had 4 eyes and had a orange tint, these fish changed designs in Halo 2: Anniversary to having 2 eyes and a teal color scheme.
The old design of the fish from Halo 2.