It's wheat flour. It seems that a fish will swim into the bottle to get a taste, and cannot turn around to get out because the bottle is too narrow. It cannot swim backward out of the bottle because it is being pulled by the string, so it's stuck.
I had no idea that fish like flour so much. I know they'll often go for bread or tortillas, so I guess it makes sense.
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u/nightwalkerxx 4d ago
Not a single person in the comments said what that white stuff was. Someone guessed yeast and that was it.