Dominance
While crayfish do not normally attack, they are more prone to attacking slow, non-aggressive fish. If your aquarium consists of goldfish, gouramis, chinese algae eaters or any other type of non-aggressive fish, it may be beneficial to introduce a more aggressive fish to the tank. While it is important that the aggressive fish does not attack your other fish, it can keep your crayfish from taking a dominant role in the tank.
Division
If your crayfish continues to attack and eat your fish, you may be forced to divide your tank into two sections. Placing a plastic divider down the center of the tank will give your fish and crayfish separate regions to live in. This will physically prevent them from fighting while in the tank. You can place your fish in an entirely separate tank from your crayfish if your original tank is too small for a divider.
Downsizing
If your crayfish is eating your other fish, the crayfish may be too big for your tank and for your fish. If you can return your crayfish to the store or place it in a separate tank, you can replace it with a smaller crayfish that may not attack your fish. A large size differential can often help deter the crayfish from attacking the other animals and fish in your aquarium.
Protection
Crayfish are known to jump out of their tanks if not properly secured. As a precaution, placing a mesh net or covering over the top of your tank at all times will help keep your crayfish in the tank. In addition, placing rocks and other fixtures at the bottom of the tank will help give your crayfish adequate hiding places. This will make it feel less threatened in the tank, possibly mitigating aggression.