Belly up cory, dead?

[Sciencelover5]

I was eating dinner and looked up to see one of my 3 pepper cory catfish float belly up to the surface of the tank. His stomach was bloated and it looked as if he swallowed a rock. There is redness around his gills. I have 3 peppered corys, 1 albino cory, and 6 neon tetras in a 10 gallon tank. The water is a constant 78 degrees Fahrenheit and I have a tetra whisper 20 filter. Just checked the water parameters with Tetra 5-in-1 easy strips and everything appears to be just fine. Just did a water change 2 days ago.

I have the cory quarantined in a small betta cup, floating in the tank but it looks like he will die at any moment

Update- in the time it took to write this, the cory has stopped breathing :'(

All my other fish seem to be doing alright.

Does anyone know what could be wrong and what I can do to keep my tank healthy so that my other fish don't suffer the same fate?

Water parameters:
Nitrate 20ppm
Nitrite 0
Total Hardness 150
Total Alkalinity 40 -80ppm
pH 7.2

 

[FishGuest5123]

Have you noticed his belly prior to incident. It could be several things...unresolved constipation, dropsy, or over eating. Check your ammonia reading.