Castellanos competed only in the men's 100 m freestyle at the 2000 Summer Olympics in Sydney. He received a Universality place from FINA, in an entry time of 53.43. He challenged seven other swimmers in heat three, including Trinidad and Tobago's 16-year-old George Bovell and Seychelles' three-time Olympian Kenny Roberts. He rounded out the field to last place in 54.06, just 0.63 seconds below his entry standard. Castellanos failed to advance into the semifinals, as he placed sixty-fourth overall in the prelims.