very sick betta fish

[spaceone11]

I inherited a betta last December and made its home a 1/2 gallon bowl. I fed him pellets and changed his water weekly w/dechlorinating solution. He has been healthy and happy until I brought home another fish from a pet store (long story but a replacement for mine), put him in my fish's bowl and mine in a smaller one temporarily. The new fish died almost immediately and mine has not been the same since. I cleaned the bowl and put my fish back in his original home but he stopped eating, became lethargic, has fin rot and possibly some ick. I took him to the pet store and they put some antibiotics in the water and gave me some TLC which I was already using. They told me he was old and probably would not live long anyway but I keep thinking there is something I can do!!! I know I did a lot of things incorrectly but he was happy and fine until recently and is getting worse.

Can anyone help me please??!! Thank you.

[Captain Crab]

It sounds too far along to do anything now.But a larger living quarters wouldn't be a bad idea.living in a fishbowl can't be good for any fish for very long.Im not totally sure about this but i believe bettas need a constant temperature.So i don't really know why they keep them in fishbowls.A drastic drop in temperature could cause stress and disease.A goldfish is a different story.Bettas i believe are tropicals being from a warm part of asia.I would buy at least a 10 gallon tank and heater.That might be the best solution.