I agree it doesn't sound like typical fish TB, which, as mentioned, would have some combination of curved spine (possibly appearing prior to illness), sores, and wasting (very quick loss of weight).

It sounds more to me like it may be a septicemia, or a combo of ammonia and bacterial infection (ammonia can also cause hemorrhagic streaking, and weaken the immune system allowing opportunistic infections to take hold). I also wonder if salt may have made the cories a bit more vulnerable, but if there truly was a good timeframe between that ending and when they became ill, it's less likely.

Keeping MI/MII or, more preferably, kana on hand (and even a kana-based med food, like Goldfish Connection's MediGold) may be wise.

~Ash