The breeding tank (named only in mission orders from Adam) is a room in Sector 4 in Metroid Fusion. It is a room in which Serris, a specimen Samus was sent to terminate, often returned to before being infected with the X Parasite. Interesting to note is that the breeding tank seemed relatively small for the large Serris, giving the impression the creature had restricted movement in it. It's unclear why the Serris was placed in a breeding tank, as no other specimens of the species are ever encountered- this may indicate that the Serris was capable of asexual reproduction.