Mysterious Sterba's Cory death?

My 5 (4 now) Sterba's Corys live in a 30gal 3ft long tank together with 6 Black Phantom Tetra and 6 X-ray Tetra.

The tank was started up in June/July this year so it's been running for a while now without any problems up 'til today.

I finished stocking the tank in September so whatever killed my Cory wasn't from a newly introduced fish.

At 2:30am they visually all seemed fine, I didn't notice anything out of the ordinary. They were fed and all of them came out to the front of the tank to feed as usual.

At 6:30am-ish I spot one of the two females on her back on the substrate with a red circular shaped hole on her underside - though I did see two of the other catfish trying to feed off of her - and a dark orange patch on each side.

I took her out to examine her and only where the orange patches are, her scales are raised in like a chevron shape ( ^ <-- that) pattern.

My water conditions:

Temperature: 24 C

Water is half treated hard tap water, half soft bottled water. (I'm not sure on the overall hardness, I think it's about 12-16 degrees)

Ammonia: 0ppm

Nitrite: 0ppm

Nitrate: 20ppm

The last water change was last Saturday (the tank's due for another change this Saturday)

I have no idea what could have killed her, but I think it might be something internal. I'm monitoring the remaining catfish just to make sure they're healthy. Any ideas?