Hi Everyone,
I haven't been on the forum for a long time

Anyways, I wasn't sure if I should post this here or in betta care, but I am looking for some input either way, as to if there is anything I can do.
I have had Sumo for a bit over a year now (no idea how old he was when I got him) and he seems to be deteriorating, very slowly. Is this just him aging and getting old? I know some of you have had bettas who have lived quite long, and I have another that I've had just as long and he looks healthy as ever, so I really don't know...
His fins are ragged (not fit rot, no decor could be doing it either, they just don't look like they used to..) and he just looks sickly and thin. He eats but the process of it seems to tire him, he is not nearly as active as he used to be but he still swims around sometimes. Most of the time though he just floats there (not on his side or anything though) and doesn't move, he just seems so weak :(
He shares a split tank with Shiloh who is very active, always has a bubbe nest, and swims around constantly and flares happily all day (rarely ever at Sumo though, I actually think they kind of like eachother), so I also don't believe it is a water quality issue or stress from the neighbour either...
So.. Is my old man (as I call him now) at the end of his days, or is there anything else I can try doing? My very first betta died right after purchase, he was sick when I got him, so Sumo is technically my "first" that I have had for awhile, so I really don't know what happens when they get old...
Any input = awesome
& here are the specs
Housing
What size is your tank? 10 gallon split so he has approx. 5 gallons
What temperature is your tank? 78
Does your tank have a filter? yes
Does your tank have an air stone or other type of aeration? no
Is your tank heated? yes
What tank mates does your betta fish live with? shiloh (male betta) on the other side, no other tank mates.
Food
What type of food do you feed your betta fish? Hikari betta pellets
How often do you feed your betta fish? 5 pellets a day with the occasional fasting day
Maintenance
How often do you perform a water change? approx. every 2 weeks
What percentage of the water do you change when you perform a water change? 40-50%
What type of additives do you add to the water when you perform a water change? prime
Symptoms and Treatment
How has your betta fish's appearance changed? skinny, looks frail, colours aren't as bright
How has your betta fish's behavior changed? more lethargic, he seems weak, rests a lot..
When did you start noticing the symptoms? it's been a very slow process over time and nothing has helped him
Have you started treating your fish? If so, how? did try QTing him a few months back but it seemed to make him worse so i let him back in his tank within the same few hours...
Does your fish have any history of being ill? not besides this slow deterioration
How old is your fish (approximately)? i have had him for over a year so i would guess he would be 2 or older since he was full grown when i got him