What killed my crab

[EmergencyRn]

Put a emerald crab in today to help with algae. Within 4 hours he was dead. Noticed he went into his cave and never came out. When i took him out he was missing a leg. Could he have died from trauma? Could a mantis shrimp kill him?

Current water parameters:
temp 78.3
amm .3mg/L (little spike, was zero)
nitrate 5mg/L
nitrite .1mg/L
Ph 8.1
phos zero
Kh 120mg/L
Ca 400
SG 1.024

noticed my alk has dropped. Was 140.

 

[supertech3]

why do you have any ammonia? Did the emerald crab die or shed? they can sometimes look so much alike What fish do you have in your tank?

 

[EmergencyRn]

Tank about 2 months old. Going slow. No Fish, just 1 turbo snail and Cerith snail. 1 zoo colony, 1 small xenia, and small acan. This is a 2.5g Pico, so i know water conditions need to be close to perfect. Dont know why i have a spike in ammonia. Was running zero for 2 weeks after cycle. Currently have small diatom bloom and some green algae, but not bad. The turbo snail is crapping like mad, could this raise my ammonia?

 

[supertech3]

How often are you and how big are your water changes?

 

[EmergencyRn]

Doing 20% water changes weekly on sunday.

 

[amike5]

He may have just molted. Crabs usually molt shortly after being introduced into a new system. You will rarely see your emerald because they are nocturnal and very good at hiding. On the other hand, you shouldn't have ANY ammonia or nitrites in your system. How much live rock do you have? How long did you cycle your tank?