Info: I have a 55 gallon tank where they all live. I'm so new I don't own or understand tools or tests for ammonia, nitrate and ph. I vaccuum the gravel and replace 1/4 of the water (conditioned, of course) every week at least. I have such a strange bunch because i got them when they were small and thought they'd be okay, and they were. I am already mulling over my options for when my koi grows (get him adopted, dig a pond...), but he's small enough to be comfortable for the time being. I have no idea about the pleco. I know it's not enough info but he's brown. I'll try to identify him another time. Under normal circumstances (as in every single day previous) they all get along. No nipping or chasing, they all get enough food without intervention.
UPDATE: I went on vacation and left my fish with a feeder cake thing that surpassed the time I was gone. But when I returned the food was gone... and so was part of my oranda! On the bright side his wen is trimmed, it was eaten around his eyes and the back/top. He can see now. I'd say less than a third of the whole wen is missing. His front fins were nibbled too!
He didn't seem to be in shock, swimmingly normally and blubbing in his usual patterns (I have watched a stressed fish die, I know what it looks like. he didn't act like that). I cleaned a 35 gal tank (no prev. disease, vinegar & lots of rinsing), got the water temp right, and moved him in alone. He seemed a little confused but not stressed. I had no gravel clean tonight but i gave him a moss ball and some ripped up, rinsed romaine lettuce. That and the bubbles from the filter should hopefully keep him happy through the night. I can only hope he makes it, I'll keep you posted.
On a final note, is there anything extra i should put in the water to help him heal and fend off infection? I'm sure there is but I don't know what it is.