[1000 year old fish not in my codex when is scanned them]

I used a guide to help me get all 20 of the fish, I have completed saiyas vigil, but the plains of eidolon fish isn't in my codex, I went around the map a few times to make sure I scanned them all and I know that the fish that unlocks the plains of eidolon codex entry is the one in the middle of the big lake on the bones, but it won't let me scan it again (I used the regular codex scanner not simaris or helios) my user in game is connordddd. Thanks