Reaching the end of the road?

[Cyanotoxin]

I've had a veiltail named Veil for the past year and a half. About two months ago he contracted some finrot that ate about half his tail away. I dealt with the problem with the recommended salt quarantine treatment and it worked just fine seemingly. His fins started growing back.
But they stopped about 3/4 of his original tail length. I figured, ok, he'll just have shorter fins now.
During the sickness he also lost a lot of color and never gained it back despite otherwise being back to normal. I figured, ok, he'll just be a short finned silver fish now; as long as he's happy and healthy I'm good.
So about a month ago I would say he was back to normal other than being gray and having short, but otherwise healthy fins (maybe a little stiff but not clamped).
Well today he's no longer alert, sits at the bottom of the tank, and when I threw a pebble in front of his face when he came up for air he took it but then spit it back out and sank to the bottom of the tank. Seems like symptomes of a cold tank but my whole house is 80 degrees, his neighbor is perfectly active (his neighbor has always been kept in identical conditions and is the same age) and the other divided tank's 2 residents are all good with the temperature and almost identical conditions (I do nothing different but it is a different tank). Ammonia, Nitrites, and Nitrates all stayed clean (I test weekly). He also seems to swim in short bursts, as if it winds him and just sinks, not swims, but sinks to the bottom tail first.

Any thoughts? Was his immune system just too compromised to deal with whatever the problem is now after that finrot episode a month back? What could general lethargy and loss of appetite mean in warm environment?

Pics for visual reference. The first is him currently. Gray. He changes from light to dark gray depending on the day of the week.
The second pic is him just after adoption. He looks a lot like he did as a youngster now, oddly enough.
The third pic is him at what I would say was his prime. Long tail and bright colors.
The last pic is just of his house. There's a whisper 5-10 gallon filter behind the dividers.

[lvandert]

does he have an unusual bump near his tail or is his body curved at rest? How recently was the first picture taken?

[Cyanotoxin]

The picture is from just a few hours ago. There's no unusual bump at the tail and he is usually straight at rest except for a very minor curve of just the tail.

[lvandert]

ok its just the picture playing tricks on me then. It's possible his immune system took a hit. It also might just be his time. I would maybe get him into a qt tank and give him a shot of stress coat and aq salt. See what it does. he may perk up but he may not. If he does perk up then there are a few things that could be going on we just can't see.

[thekoimaiden]

If your house is 80F his water is no warmer than 77F as air temp is always warmer than water temp. Do you have a thermometer to tell us the exact temp? Is there a fan or anything moving air around in the room and facilitating evaporative cooling? My betta tanks will drop to 76F without a heater in the summer despite the fact that my room is a wonderful 80F.

[Cyanotoxin]

First: yes, the house is 80 on a cold day (usually 82 since I try and save money on electricity and get cold easily myself anyway) and I measure to water to be consistently around 78, 79 if the tank light is on all day. We have high humidity and little air movement. I don't think it's a temperature issue mostly because the other 3 are active as always and because it's been this way his whole life so I can't imagine him getting cold all of a sudden from the same temperature. Nothing is out of the realm of possibility though and if older fish need water above the 80s then it will be done.

Second: I did notice some clearly noticeable vasculature at the base of his tail. Just took the pic below. Is this normal? It doesn't look ruptured or anything but I don't think he's ever been translucent enough to see veins through.