you know, it's funny you just asked that.
milalic finally let me adopt a pair of his,
and within a week one had a claw missing,
so even 2 in a 20gal tank can be a problem.
the point is, the more ways you design your
tank bottom to keep crayfish out of each
other's way, using tunnels not caves, so
they always have an escape, the better
off they will be. a great idea I have for
dwarfs is to carpet at least half your tank
with small lava rocks. the crabs can then
hide in the cracks and valleys between
the rocks, so they can always hide from
any particularly aggressive individual.