I'm thinking two things...(1) the tank did not cycle properly. Beneficial bacteria need ammonia to feed on and in not sure a few crabs put enough off to do the job. (2) you added too many fish, too soon and overloaded your bioload. I go with a fish, maybe two, per month just to allow my BB to adjust to the increased bioload.
What are your parameters now? Have you done a water change to bring your nitrates down? I suggest getting a good test kit. Strips are notoriously bad for accurate readings. If you have any Ammonia or Nitrites, your tank is not cycled.