Lethargic Bronze Cory Lying On Side

One of my bronze corys (which I purchased a month ago) is being very lethargic and lying on it's side. It has been like this since yesterday. I have tried feeding it a pea, but it seems to have low energy rather than buoyancy issues.

Water parameters are fine in the tank, 0 ammonia, 0 nitrite, 10 nitrates and a pH of around 7.5. I haven't added any new stock since I added the cory. The only other issue I have had in the aquarium is the gourami eye issue (https://forums.thinkfish.co.uk/fish-health/gourami-with-cloudy-eye/msg50311/#new), which I strongly suspect is an injury as it hasn't affected any other fish and only the 1 eye on the gourami.

Any ideas what could be wrong?

Thanks

I have 3 more bronze corys and all are behaving normally. I last did a water change on Sunday, I do water changes weekly of around 25 - 30%.

I don't use CO2 at all in my aquarium.

The cory is in quite a bad way so I have moved it to a small tank with a low water level for now so it can still get to the surface to gulp air.

Thanks for the helpful comments.

Unfortunately the cory died this morning 

I have absolutely no idea what was wrong.