fish on bottom for a week, normal a week, now on bottom

[deemoney]

I posted about a week and a half ago about my white tetra. He was laying on his side out stomach, and would swim for a bit then rest again. The first day I didn't test water just went ahead and did a 50% water change. Next day tested water in morning and later that night, checked it twice a day for next three days. I even took water to an exotic pet/fish store to be tested. Water is perfect. He is approximately four years old. Manite told me they only live to be four or five. Told me it's most likely old age. I'd been wanting to add more fish to my aquarium but decided to let him be in peace not stress him out with newcomers. It's a sixty five gallon tank and it has only e him and two black tetras. Tank has been going for about three years now so it's cycled, all tests say water is perfect. Any how, he was like that for a week then was back to normal for about five days. When I woke up today he's back on his side. Still eating, he's worse than last time, kinda just drifts and lays so still I thought he'd died. Plus now he has a slight bend in his body just behind his head. Does anyone have any idea? I've had him so long, do what I can to save him, keep him comfortable. Thanks for reading, I'm kind of long winded, sorry, just want to make sure I don't leave anything out. Again, thank you all!