+1 tank over stocked. why asking on temp range? they like between 76 and 79 , but do fine higher up to 85 or so, but you put it forth like there are large temp swings. this is not good, Oscar can be picky eaters when you change things up, like food. he can go 3 weeks easily with out eating, if its just a change of food thing then he will eventually eat, no happy stress free oscar in good water quality ever starved them selves to death, just a mater imprinting this food now. I am more curious as to water change schedule and perams are running. that stock looking at a 80% water change every other day, tank dosed with prime for size of tank not water replacing, until you get a bigger tank this should be the norm. I say every other day because prime binds for 48 hours. keep in mind binds, does not remove.