Crayfish are fun pets! They can be set up in a normal aquarium and they are easy to maintain as pets. They are also more interesting than fish in many ways. Due to their different body type and physiology, they can do things that fish cannot do- like grab things and climb. You can spend hours watching them. They are nocturnal creatures, coming out to feed at night. They don't like bright lighting. If you are going to use an aquarium hood lamp, then use a colored bulb or a low wattage bulb.
Crayfish do not share tanks very well. The largest one will usually try to establish itself as the dominant crayfish, and will fight, injure, dismember or kill other crayfish that challenge it. Crayfish also have the mindless tendency to climb on each other, and they are sometimes injured when the other animal gets annoyed by this.
A cheap, easy solution is to partition the tank. Partitioned tanks can share the same water filtration system, and you can do water changes and tank maintenance to only one tank, instead of three. We had divided our 10 gallon tank into 3 separate sections, each section housing one crayfish. There are tank dividers that you can buy, but they can get expensive ($10.00 per divider), and the divider sheets have holes that span the entire sheet. This is very disadvantageous, if you intend to separate crayfish from each other.
A cheaper and better solution is to make your own tank dividers, if you're feeling creative. Our local plastics supply store sells clear plastic runners- long rods of clear plastic bent to a "U" shaped groove. We measured the inside of the tank, and asked them to cut 2 sheets of clear plastic, exactly 1/4" narrower than the tank width. Then we drilled holes in the sheets to allow water to flow through all 3 tank sections. Make sure that you drill the holes only half way up, because the crayfish WILL use the holes as footholds to climb. If they can, they will climb into the other sections, or out of the tank. With the holes drilled only half-way up, the crayfish cannot completely climb over the partitions, but they will constantly try, which is very entertaining for us to watch. Cut the plastic runner to 6" pieces. Use waterproof sealant or adhesive to glue the runners to the bottom, and to one side of the tank. Then slide the plastic sheet between the runners. The sheet is not secured on the top edge, and on one side of the tank, but that's not a problem- the crayfish will be unable to move the divider sheets, but you, the owner, can easily remove them to clean the tank.
We prefer to leave the tank gravel-less. Gravel makes it harder to clean, and gravel would interfere with the plastic runners.
Next, the crayfish need housing, and something to do when they are not trying to climb out of the tank. This can all be accomplished by making some "crawdad tubes" for them. The materials are cheap, and better suited to crayfish than anything you can buy in an aquarium store.
Buy a PVC plastic "elbow pipe joint" at a hardware store. Choose one with a large diameter- like 2". Use silicone adhesive like "GOOP" or aquarium sealant and glue pieces of aquarium gravel to the outside of the pipe. The gravel serves 3 purposes- it makes the pipe look like a piece of aquarium decor, and it provides crevices that the crayfish can use to look for food, and to climb on top of the tube. Yes, they enjoy climbing on the pipe and perching on it. Then, you can drill a hole in the top of the pipe and put a plastic plant in the hole. The plant provides the crayfish with additional amusement. The crayfish can climb the plant, or they can chew on the plant when they're bored.
The main purpose of the tube is to provide them a place to sleep. The crayfish might be "spooked" by the tubes at first, but after a day, they will figure out exactly what the tubes are for, and they will happily use them. The crayfish will crawl inside the tubes and curl up and sleep for hours in them. If you are cleaning the tank, the crayfish will scurry into the tubes for protection. The hard part is getting them OUT of the tubes when you want to scrub them. Notice the little rubber bands. That was our unsuccessful attempt to stop them from fighting with each other. Eventually they figure out how to slip the bands off of their claws.
Since we have a box filter and an airhose, we sometimes have the problem of an opportunistic crayfish climbing on the filter and grabbing the airhose and climbing out of his own section. To fix this, another small partition can be made with a groove for the airhose. Additional runners are cemented on this extra piece to fit perfectly on top of the other tank dividers. This will provide a "ceiling" that the crayfish cannot pass, even if he's clamped onto the airhose.