The breeding tank (named only in mission orders from Adam Malkovich) is a room in Sector 4 (AQA) on the Biologic Space Laboratories research station in Metroid Fusion. It is a room in which Serris, a specimen that Samus is sent to terminate, often returned to before being infected with the X Parasites.
Believing that Serris will be encountered here, Adam even pinpoints a boss icon in this room in the map data provided to Samus, but upon entering the breeding tank, she comes upon the original Serris's remains (consisting of bones only). Samus proceeds on to the next room, and finds an X mimicking a Gadora, indicating the presence of a powerful X creature. The following room is where Samus battles and defeats the X-mimicked Serris.
Interesting to note is that the breeding tank seems relatively small for the large Serris, giving the impression the creature has restricted movement in it. It is unclear why Serris was placed in a breeding tank, as no other specimens of the species are ever encountered--this may indicate that Serris is capable of asexual reproduction.
The eerie theme that plays in this room is titled Environmental Sound (Disquieting) in the Metroid Prime & Fusion Original Soundtracks.