all you can do at the moment is water changes, and this will help, thats if the water is indeed the culprit which at the moment seems increasingly likely, obviously enough crabs are carnivorous creatures and would have a feast eating lots of your fish, but i very much doubt it could catch them, especially the nippy little fish you have been loosing.
if the cichlid is breathing heavily its normally a sign of a lack of oxygen or could be something in the water irritating it i.e. ammonia and nitrite. if you can do a water change that should help and if possible turn your filter outlet so it is disturbing the surface of the water.
have you been dechlorinating the water when you do water changes? if not the helpful bacteria in your filter will be getting killed off everytime you add water, which in turn will be leading to increasing ammonia and nitrite levels.
if you can and dont mind doing id re-home the larger of the fish, just for the fact they will be the biggest waste producers and would ease off the pressure on the filter.
like mentioned we really need your ammonia, nitrite and nitrate levels and if nothing else we can eliminate it.
David
