Okay, I am asking this for a friend who has three koi (which I know nothing about), any help would be much appreciated. 
I can relay your questions to her, I know she will be grateful for any help. She had one suggestion to change the food out since it is pretty old so she is going to try that first and she will see if that helps.
Nothing has changed in their 55gal tank (they're tiny koi, so more like large goldfish). No new fish, no new food, no new treatments. I have a 80gal filter in there (because koi are dirty fish), and I do 30% water changes every two weeks (siphoning from the bottom).
Last week my large pleco died (he was several years old) and I noticed that there was a hole in his underside and green stuff was seeping from it (perhaps from his stomach?). Over the last week two of my koi have died.
I've done another water change, added a bit of salt. There's no visible fungus/parasites on the fish. They just seem to get lackluster and then die. No clamped fins, nothing. The ONLY thing I've seen one do is float with his head tipped down, but he's one of the ones that's alive and perky, so I don't know if that was him being ill, or if that was just me reading into a behavior I know indicates illness but he just happened to be doing when I was examining the tank.
Unrelated, one of my koi has gotten a white growth by his nose. It isn't cotton mouth, and nothing I've treated the tank for has ever cured it. It hasn't spread, it's just this translucent white, slightly raised bit of skin above his nose. I had another koi with something similar in the same spot (now a few years deceased). The stuff in question is on the top of the nose of the orange white and black koi in the background:
I can relay your questions to her, I know she will be grateful for any help. She had one suggestion to change the food out since it is pretty old so she is going to try that first and she will see if that helps.
