I need some advice from you all. I've had a betta for approx. 2 years
now. A couple of months ago I moved him (along with a couple of tetra)
from a 20 gallon to a 3 gallon tank. After about 2 weeks he started to
just sit at the top of the tank and not move. I diagnosed him with a
possibly bacterial infection. I moved him into a bowl and treated him
with drops for about 12 days. After about 2 weeks, he started swimming
around and being much more active, I think moved him back to the larger
tank.

He did well for about a week or so. Now I'm noticing that he is again
sitting on a plant near the top of the tank. He is still eating and
moving around but he sinks when he tries to swim. I just feed him and
now see a white string hanging from him. Is it possible that he's had
worms this whole time? I don't feed live worms in this tank but some
of the ornaments came from a tank where I used to feed black worms.
The ornaments had been out of any water for over a year and were rinsed
before putting them in this tank.

The tetras are doing just fine and the algea eater in the tank doesn't
seem affected either. Is there anything I can do for him?