emerald crabs dying...don't know why

i have a 36 gallon bowfront tank, it's about 6 months old with 55 pds of live rock.
it has 1 turbo snail, 6 astrea snail, 5 ceriths, 2 fire shrimp, 1 six line wrasse, 1false b&w clown and a blue reef chromis...
i had 5 emerald crabs in it but in the past 2 weeks i lost 3 of them and i don't know why. my water tests turn up perfect, my fish, snails, and shrimp are doing great but i can't keep the crabs alive.
do they have short life spans or something? or is there a disease that i just don't know about that only they get?


could be a couple different reasons
there isnt enough food in the tank to support that many
there having a problem when they molt * sometimes it just happens when an invert molts it is to hard on them and they die in the process*
they are fighting each other.
for a tank our small I would only have 1 emerald crab 2 tops in there.