I see discoloring on the face, but can’t get a good enough look to say why. First off, test the water, post results. API liquid pond test kits are fairly easy to get, and give good results. Next, describe what you saw prior to fish death. Any jumping, flashing, hiding, listless, loss of appetite, etc. Then describe what you saw on the dead fish that you think indicates disease.
The only thing I’d recommend being added is stress coat until you know more. Or if he is in a panic, add a bottle of peroxide, that is safe and just turns into extra oxygenated water for them. Then post the above info, and we will try to help. Meanwhile, if your water is below 45 F, don’t feed them. If it’s above that, add crushed garlic to their food to boost their immune system.
