So when I was growing up we kept a blue crab as a pet a couple of times (over several years) and we didn't have a problem doing so. I recently tried to do this again and both times I've introduced a blue crab to the tank it has died within 12 hours of introduction. I am new to saltwater tanks (except from memory). I set a 10 gallon tank up 4 weeks in advance with a canister filter (no charcoal, just UV light and mechanical filtration). NH3 levels were below readable and salinity was about 11 ppt (which is what I read when I tested the actual river water where the crabs came from). Admittedly, I never tested nitrate, nitrite levels, or pH. I used "City water" and originally added it straight to the tank. I never changed the water. Obviously, (obvious to me now) I did not monitor an actual nitrogen cycle occurring in the tank and there was possible (probable?) chlorine in the water from the start (grew up with a well, didn't think about it). Since a blue crab straight from the water can live a pretty long time out of water anyway, dying this fast seems like there was something very much not agreeable with the tank water. I am going to go ahead and do the proper cycle, de-chlorination and monitoring now but I wanted to see if anyone on this forum can think of anything else that could be wrong? Not interested in any more preventable mistakes..