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Second, get yourself an API master test kit or one like it, not the little strip tests, to test your water.
Third, leave this experience behind you and move forward, everyone makes mistakes.
Fourth, do not listen to people at stores like Petco.
Fifth, you came to the right place to get help, the people here are very knowledgeable. Opinions do differ on subjects but overall everyone basically just wants to help.
Six, do some searches in the forum for thread about crabs. There are many I'm sure and the info you will find is bound to keep this from happening again.
I've had a couple of different varieties of crabs before and in the case of loss of limbs it is indeed a bad thing however they can live without it however be careful! it may look dead and may actually be molting so unless it's been several days it hasn't moved leave it alone! otherwise if disturbed it will die. As for the loss of the limb it could have been a result of it attacking something I kno with other crabs they can be aggressive sometimes and some fish although i've only had crabs lose limbs because the were fighting with other crabs
I'd listen to zparticle too i'm not super experianced about crabs but if it is dead it could have been a a result of not acclimating properly or too much ammonia nitrates nitrites etc. However like i said just cuz it's flipped on its back and not moving doesn't mean its dead!
Your welcome hope it works out for you if it doesn't you should definitely do a little research before getting another one i've made the mistake of not doing enough research often enough lol