I don't think he's eating. Maybe stuff he's finding on the bottom corner he's hiding in tho...He appears to be at the bottom of the tank, 'gasping' through his mouth for air... Do you think this means he has an internal infection???
That sounds like an internal infection. Once they stop eating, breath heavily and do stringy white poop, it is usually over. I would euthanise the fish and monitor the remaining fish over the next few weeks.
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Do the following to reduce the risk of infection to the remaining tank inhabitants.
Wipe the inside of the glass down with a clean fish sponge. This removes the biofilm on the glass and the biofilm will contain lots of harmful bacteria, fungus, protozoans and various other microscopic life forms.
Do a 75% water change and gravel clean the substrate every day for a week. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use them. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens.
Increase surface turbulence/ aeration to maximise the dissolved oxygen in the water.