The .25 nitrites will kill fish - immediate water change when you see this to reduce to 0. Then work on establishing how the spike happened (over-feeding, death in the tank unspotted, poor maintenance generally, some disruption of the bacterial colonies e.g. cleaning filter media in chlorinated tap water...)
Have you anything capable of eating the loach whole in there ?
If not, she's in there somewhere alive or dead (under a rock, driftwood, behind a filter pip/heater). You'd need to take absolutely everything out to be sure she had magically vanished.