with crayfish you might need to add iodine.. i've actually found quite a few things places that recommended this if it looked like the crays were having trouble molting.. add at half the recommended dosage for marine aquariums.. something like a teaspoon for 50 gallons (this isn't regular "iodine" that you get at the drugstore, it's what i got at a marine aquarium store, and i think is labeled iodide-i'll check it later)