Oscar of 1 month died. Why?

Accidentally posted on the cichlid section -.- lol its 4 am nd im tired. Bf found my oscar dead. He had been eating fine and begging so dont know why he died :/ tanks been up and running for a year
Nitrate 5-10 ppm
Ammonia .25
Nitrite 0 ppm

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Ahhh my stepdad said he saw a fish jump out when he came in :/ he put it back and it was alive. I had forgot to close the lids :/ now I feel stupid and more sad -.- first time keeping an oscar

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2 severums. 5 silver dollars. 1 convict. And a port cichlid. And yes I was going to rehome fish to make room for my oscar and already had an ad for craigslist going -.- the convict is female and leaves the others alone. The only one she has attitude with is when the wild sev tries to mess with her since he dominates the tank. I recently added the port but he had been in a small tank with big oscars and dempseys and had been ok so i figured when i rehomed the other fish they could be buddies Does it look like a fight? He wasnt torn and knowing my step dad im assuming he put him in the tank with his hands

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Stepdad just told me the story himself. He got home and saw something black on the floor. He pushed it a bit with his foot and it moved. He threw him back in and said it looked like he might have been out for a while :/ out of all the fish -.-

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