Crayfish can prove predatory and disruptive when sharing an aquarium with fish. Crayfish are enthusiastic omnivores and will happily devour aquarium plants, fish and other invertebrates. Exceptions exist, but generally, you have the greatest chance of success if you keep crayfish in their own dedicated aquarium.
Crayfish will attack and eat other fish if the crayfish can catch them. Most fish move too fast for crayfish during the day. However, in their nightly torpor, these same fish make easy prey. Smaller species of crayfish may have a greater chance of peaceful coexistence. Additionally, small, fast-moving fish like tetras stand a better chance of escape. But the bottom line is that any situation where crayfish share an aquarium with small- to medium-sized fish stands a good chance of turning into a buffet for the crayfish.
Conversely, large fish may prey on the crayfish. Many larger aquarium fish consider crayfish a delicacy. Oscars and other large cichlids, for instance, will consider crayfish a treat. It can prove difficult to find a fish that's large enough to avoid getting eaten by the crayfish, but small enough that the fish itself does not pose a danger to the crustacean. In fact, some pet shops sell crayfish as live food for larger aquarium fishes.
Additionally, it sometimes seems to delight crayfish to disrupt the decor of fish tanks. This is not a reason not to keep a crayfish, but it does effectively ban them from a carefully-decorated aquarium. Crayfish will enthusiastically eat and uproot aquarium plants. Crayfish also like to burrow to search for food, hide themselves and cause trouble. This behavior will knock down any rockwork or aquarium ornaments.
Additionally, crayfish require some care above and beyond aquarium fish. Crayfish love to escape from aquariums, so you need to secure the lid tightly and make sure there are no gaps the crayfish can escape through. Additionally, crayfish are extremely invasive, having already disrupted numerous waterways. While you should never release any aquarium pet into the wild, crayfish are of particular concern due to their adaptability. Also, if you plan on keeping more than one crayfish, you need to make sure they have deep enough substrate to bury themselves to hide from each other. This can help avoid cannibalism.