i have had my 5g nano running for about 5 months now. i added 2 snails 3blue leg crabs and 2 red leg crabs. i bought four coral frags yesterday and put them in the tank. 3 heads of duncans a flavia sunset monti and a pice of merulina. the corals are all doing well and the snails are still speeding around the glass but all of the crabs are dead. so i broke out the test kit and went to town. no ammonia no phosphate and my ph is 8.2. I'm not sure what happened. everything is still alive and showing no signs of stress. has this ever happened to anyone else? does anyone know what could have caused this to happen? thanks