I lost a discus(red turquoise) once. Before I don't put any covers in my aquarium just because I wasn't aware that discus could jump! My routine is to count my fish (since I am new to the hobby
) first thing in the morning, if there are deaths etc.. They were supposed to be 9 but I only counted 8! wtf!? I thought someone must have robbed me! haha later on I found out that my missing fish was stuck at the back of the aquarium. (I assumed of course it jumped) since that day, I always put covers in my tank just in case.