[zenyfish]

I remember you kept it with a loach. Loaches are much more aggressive than plecos, and depending on size, could've killed the bottom-dwelling competitor. Watch for signs of aggression.

 

[zenyfish]

Even if there were no physical damage, a fish can die from stress alone, especially if it's already stressed from being in a new environment.

Loaches are nocturnal, so you may not see their aggression during the day. Loaches have spines under their eyes they can slash with. Not to say that the loach is the guilty party. But I would not keep a small pleco with a larger loach.