Would crayfish do well with koi?

Sometimes fiddler crabs or other crab species are sold for aquariums. Most require brackish or even saltwater. Few thrive in freshwater. Most require live insects, worms, and/or small fish to eat. They can harass or kill other small animals in the tank with them. Many have nasty pinchers. For this reason, they are best suited to a set-up solely for them. Almost all of the crabs sold for "freshwater" tanks require a haul-out spot. The water level must be low enough to allow them to get in and out of the water with ease. If denied the ability to get out of the water, they will eventually die. They most likely also require full spectrum lighting over the basking site but studies probably have not been done on this. Fiddler crabs and similar species stay relatively small, at a few inches in size.