well a 55g is not a lot of swimming for these fish...
however seems you may have something in your tank that you are not aware of. what could cause the bite marks?
if it had discolorations on it, describe them because that is suspect to its death and behavior. could be a fungal or bacterial infection or something like that. sometimes the YTs are very aggressive. I have had the very rare case of a YT that ate all of the antennae off all of the shrimp in my system. seen it posted about 3 times in my time on here. some open ended questions here that need to be answered, did you by chance take any pics?