Change in Swimbladder

Since November 15th - we brought in a fish that was swimming upside
down in our small pond outside and put him in a 29gal tank.
Up until a week ago, he was swimming upside down able to move about the
tank - but not able to keep himself down in the water for very long. He
was eating (although I never actually saw him eat). I feed him every
few days - shrimp brine or a pea or sinking wafer.

Now... he has started sitting at the bottom of the tank upside down.
Occasionally he moves around. But, he seems very lethargic to me. The
Ammonia,Nitrate,Nitrite levels all test zero. The ph is around 74-76.
The tank temperature has been around 68. And it has been cold lately
and the tank is near a window. I put a heater in to raise the temp to
the low 70s. I also added 4T of aquarium salt.

My question is... why the sudden change in flotation? Could it be
affected by temperature or does this signal the end for our fish?


Thank you for any help you can provide.
Suzanne