Not sure if my fish is sick or just old

Hi
I'm new to the forum. I have a swordtail, he's been with me ever since I started the hobby 4 years ago. All his tank mates have come and gone and come and gone. For some reason he's the only fish that never caught a disease or had a single issue. He's lived a good life chasing tail, eating well, and swimming around. I recently had an ich outbreak, he never caught it but all the other fish did. I took out the carbon filter and used apI itch cure.

After the rest of the fish were cured. He was fine for about a week. I used salt for the ich. Now he just goes to the corner and sort of sluggishly swim/lays. He will come up and eat like a king as always but goes back to the corner. All water tests have been done on a frequent basis with nothing out of the norm for him. He's probably about 4.5 years old. Is it just an age thing? He has no spots, no discoloration, no bloating. He just eats and goes back to the spot. He will come up and swim around for a bit sometimes but he has always been the most active fish out of every fish I have owned. The guppies I have chill with him like hes a shark and the shark have those small fish that clean the shark.

After a while the guppies leave they used to swim around with him but now he doesn't move.

My tank is 40 gallons he's always been in the tank. Is it an age thing or a disease? I have a feeling its age but just sad to see him go as he's a survivor.

If anyone has witnessed a fishdying from old age would be helpful or if anyone thinks this maybe a disease.

thank you!

 

Sorry I meant to say I used apI itch cure after the itch had gone I changed water with salt and since then have done another water change.

 

I’m not 100% sure as I’m not super familiar with swordtails but just looking up lifespan it says 3-5 years so I’m leaning towards old age. Maybe check his poop to see if he has internal parasites to account for his fatigue but with no outward signs of disease it seems like just age.

 

Poop looks normal, brown blackish. Thanks for the reply. Will continue to monitor and update. I'm thinking its age as well. On another note does anyone know of swordtail or smaller sized fish that can live like 20 years plus? is there one out there?

 

Update:

I decided I would observe my swordtail in the dark. to my surprise he becomes super active. I've since dimmed my planting lights and the old guy is somewhat back. Not as energetic as before but better. Didn't think age would affect what sort of lighting he likes but if was definitely the light he didn't like. I've since experimented with my lights and he doesn't like the brightness I had it on for the 4 years he's been in the tank anymore.