If the ammonia was high, the other tank mates could not have been fine.
If the RTC was the largest, it can be explainable. No running water = little or no oxygen. The largest fish have the highest DO demand. They often go first. RTCs are also a bit dumb IME not always coming up to the surface to breath more aerated water.
Not enough oxygen = brain damage (brain is starved of oxygen), then death. Even if it comes to, the brain damage is usually irreversible. One of the symptoms is the fish cannot tell up from down or any sense of direction.
Ammonia alone if it was less than 4-8 ppm, would not kill so fast.
If pH crashed and the rtc went into shock, it may recover.