Weak Betta

Question
I have a red betta that I have had now for a little more than a year. I keep him in a 1 gallon tank with an oxygen stone. I change the water about 2x/month. He eats about 3 pellets twice a day and has another betta tank next to him of the same size and shape. I hardly even turn the light on because he seems scared of it and he gets enough light from a nearby window. I am worried that my betta's tail is getting too heavy or something. He seems to be always at the bottom of the tank. When I feed him he struggles to get to the top of the tank and it appears that his tail is like an anchor. He will swim to the top and then his tail brings him back to the bottom. Is there anything I can do? My other betta is quite active and has no problem swimming around but this one is scaring me a little because he used to always be active.

Answer
The personalities change as the grow older, nothing to worry about until he stops eating.

Also, take out the air stone, there is no point!
Bettas are labyrinth fish, which means they can breathe pure air right out of the surface through a modified lung. All the bubbles do is scare the betta, they come from very slow moving areas and too much movement would stress it.