Oscar Fish death with no explanation

[PrestonLandolt]

I got an albino oscar fish to add to my
75 gallon aquarium. Currently stocked with a 2 year old, blind Oscar. Only 6 or so inches long.
The new fish we brought home was roughly 3 or 4 inches long and appeared healthy. He looked like he was doing well after temperature and water acclimation.

I watched him For about two hours, (no aggression whatsoever between the two fish), left for an errand, came back home two hours later to find him still on the bottom of the tank.

Ph: 7.6
Ammonia: 0
Nitrate: 0
Nitrite: 0

He has absolutely no wounds or anything on him so killed by the other fish is unlikely. Since my water is nice and I acclimated for around 40 minutes (maybe a little less or more I wasn’t counting), I have literally 0 clue as to what could’ve killed him.

Anything helps, thanks.

 

[Brookster123]

How old is the tank?

 

[PrestonLandolt]

How old is the tank?

Been up and running for just under two years

 

[earhtmother]

Water change/cleaning routine??
Tank "contents"- gravel, ornaments, filter.....??

 

[PrestonLandolt]

Water change/cleaning routine??
Tank "contents"- gravel, ornaments, filter.....??

Here’s a picture

Water changes around probably 2 a month, I should probably do it more often, but I mean with the good water quality it shouldn’t be harming the fish enough to kill it
I get rid of algae off the sides and front but don’t bother with the back

 

[PrestonLandolt]

@earhtmother any thoughts?

-I wasn’t sure if you could @

 

[Brookster123]

Sometimes fish die. It happens. If your water tests out fine and there is no apparent cause then its just a random death.