My 55 gallon fish tank is cycled and not new. I have koi fish in my tank ( for now untill my pond is finished ) A couple of weeks a go I got a new fish from petco, I put him in my tank ( after letting is bag sit in the water ) and everything was good, he wouldn't eat for a couple of days which I thought was pretty normal since he was new to the tank, well then I noticed my other fish were not eating and they all had their fins clamped, one of the fish had this white fuzzy stuff and him and new it wasn't good so I treated with melafix for 7 days and the 8th day I did a 25% water change like the instructions said. I also took the filter cartridges out but left the filter on, like the bottle said. Well the next day two of my fish were dead. and a couple of days after that the 3 surviving fish ( 2 old ones and the new one ), started eating. This went on for a couple of days, their fins were somewhat clamped but I thought it was fine. So yesterday I did my weekly 15% water changed, treated the tap water and added it to my tank. Today my ammonia levels are between 1.0 and 2.0 ppm. My fish are stressed out and I don't know what to do. Also I don't know if this is bad, but when they come up to the surface to get food and go back down in the water bubbles come out of their gills. Please help and any advice I would be extremely grateful for. I also have a quarantine tank that is cycled, should I put my fish in their until the ammonia in my 55 is takin care of?