i noticed no lesions , let me take pleco out of freezer and re-examine

Im puzzled my royal collection of four has been mysteriously dying off this week. Only fish in tank affected . Lost my third one in three days only full spot L330 left. Any ideas? thank goodness I placed my Thunder pleco in another tank


1. Water parameters
a) Temperature range. 81 F
b) pH. 6.5
c ...