The ritual for summoning the possessed elder guardian isn't accepting any fish that I could find. Cod, Salmon, Tropical Fish were attempted. Modifying the ritual with a datapack from forge:fish to forge:chicken allowed the ritual to complete just fine. It seems to be an issue with fish.

System (please complete the following information):

• Occultism Version: occultism-1.20.1-1.109.0
• OS: Windows
• Minecraft Version: 1.20.1
• Modpack Link and Version, or list of mods: Allthemods 9 and Allthemods 9 no frills

Additional context
Atm9 implemented custom ritual designs but that is not the cause of this. I also tested it without the modifications to rule it out.

Closed in #1043