Yeah I didnt think of how disorienting it would be especially going from dull unnatural light back to sunlight. He is probably telling stories how the big creatures abducted him, kept him in a small puddle and just stared at him for days. Poor guy But yes he is acting normal and eating very well.

Yes, I agree with the liner. I would reccommend a Koi specialist come in and transfer the Koi to an oxygenated invironment while the pond is drained and the liner can be installed. Put in some new air stones with an aerator. If it does not have a UV filter, I would reccommend one of those as well. Roots are OK for decoration as long as they are bleached by the sun until dry and gradually introduced into the invironment.

I have a indoor 125 gallon pond with two Koi. Both very healthy. Water changed three times a week. They will be Bonzai Koi.They live with a lion head goldfish, 4 Gouramis, 1 Bala Shark, 2 long skirted small tetras . All get along fine all are healthy. Now back to what I wanted to say. I have a long skirted Tetra that did not eat for a month. Have a few plants in the pond so I thought maybe she was eating some of those. Never saw her though. She is on the skinny side. Did not seem to be sick, not hiding, swimming with the others. I only use salt if my fish get sick. I have never had success with antibiotics ( do not reach for over the counter meds at all) (If it is the fishes time to pass on I let them) (For me this has been rare once fish established). All the fish died when I use to reach for antibiotics. Now I keep clean water and do water tests. I do keep a jungle food antibiotic on hand for those still eating ( fish has to eat pellet, so if not eating then it is up to fate). Right now I never keep salt in the water. Only use it when sickness is found, and only in a small 10 gallon pond that works as a quarantine exception would be if ich was found in the main pond or more than one fish sick. Amount I use when sickness is in pond is 2% to 3% salt which is 2 tablespoons per 5 gallons or 3 tablespoons per 5 gallons. Today after one month the Tetra came up and ate some blood worms. Often the fish's immune systems heal them. I rarely have sickness. All my fish fed frozen foods, greens, and oranges , etc.... Fish have a real slow metabolism and can go a long time without food. If your fish is not hiding out, swimming with the others, acting normal, then why fill it with a lot of chemicals. Let nature do its thing. I usually just keep my eye on it.

Very stressed fish can lay on there sides like that. Or it could be more problems such as shown in the link below, I can't remember if you salted the pond to help with the nitrite problem but salt now would help a stressed fish. I would take the whole pond up to 0.3% (0.1% a day) rather than keep stressing it even more trying to catch it. Also some medicated food may help with a possible bacterial infection but it could just be stressed. Here is a link that might be useful: Layover