I am glad environmental got updated to 1.20.1 because it's a fantastic mod but for some reason slabfish mob doesn't work on this version . I hope it'll be fixed soon

if you have Alex’s Caves in 1.20.1, Slabfish will not spawn due to on issue on their end. I will try to push a fix for this as soon as I can, but for now you can uninstall Alex’s Caves or use this mod to see Slabfish again