Aquarius is a song that first appeared in Akumajou Densetsu (FC). It played while you were at the Sunken City of Poltergeists and it has since been remixed in many Castlevania games. It was also used as the theme of Yoko Belnades in Castlevania: Harmony of Despair (X360).
Akumajou Densetsu (FC)
Castlevania III: Dracula's Curse (NES)
Castlevania: Circle of the Moon (GBA)
Castlevania: The Adventure ReBirth (WII)
Castlevania: Harmony of Despair (X360)