The fish in question is the ICHTHYS
Because the early church was being persecuted they used the fish to identify believers.
Ichthys means the following.
Iota (i) is the first letter of Iēsous (Ἰησοῦς), Greek for Jesus.
Chi (kh) is the first letter of Khristos (Χριστóς), Greek for "Christ" or "anointed".
Theta (th) is the first letter of Theou (Θεοῦ), that means "God's", genitive case of Θεóς, Theos, "God".
Upsilon (u) is the first letter of huios (Υἱός), Greek for Son.
Sigma (s) is the first letter of sōtēr (Σωτήρ), Greek for Savior.
