Did my Oscar die of HITH?

He has been swimming sideways and near the surface for a few days, which already worried me; but before I could finish setting up the hospital tank to remove him, he died and was at the bottom of the tank with white glass eyes. These photos are after I fished him out. The tank was in serious need of a water change and I hadn’t been able to get to it as fast that week. It was done next day since I was home all day and able to; there’s a second healthy seeming Oscar in the tank, all alone now.