You can eat isinglass after eating crayfish, and there will be nothing wrong with it.

Isinglass is incompatible with foods containing tannic acid: the calcium in seafood foods combines with tannic acid to form a New non-digestible calcium tannate, which can irritate the gastrointestinal tract and cause discomfort, such as stomach pain, vomiting, nausea or diarrhea. Fruits containing more tannic acid include persimmons, grapes, pomegranates, hawthorns, green fruits, etc.