I recently purchased a 6 inch rhom piranha from shark aquarium and put him in a 55 gallon tank with 5 dither fish (exodons). hes been very skittish and every time i walk by he would swim really fast and bump his head on the wall. Every day i would offer him a silver side, because thats what the lady at shark aquarium said they fed him, but he has not taken it. So for 5 days he has not eaten. Also, two days ago i noticed slight fin rot and today it escaladed to the point where he almost doesn't have a lower fin and his other fins are starting to go also turning white at the tip. I did a 50% water change, i took out the plastic plants, cleaned out the entire filter, used meds for fin rot, and also used meds for amonia. I added a heater and took out the exodons. I'm supposing its from all the stress but is there anything else I can do? Or am i doing something wrong? Any advice would help. thanks! 
