In the past couple of days, our goldfish has been acting weird. It's area underneath it front fins has swelled. I looked up Swim Bladder Disease and Constipated Gold fish and it does not appear to be the problem as they are not swimming upside down. We introduced a new fish(scavenger) about two weeks ago and it died. Since then the fish has become bloated and is hanging out on the bottom of the tank. What might the issue be? We think that it might becoming sexed as it is aobut 1 to 2 years old. It looks healthy otherwise. What should we do?

Did you quarantine the new fish before adding it to the tank? It's possible the new fish brought a nasty into the tank and has infected your goldfish.
The scavenger may well have been sickly for some time before you purchased it and the stress of coming home caused it to die.

Is the goldfish still eating? Once a fish stops eating you know you are in real trouble, but if he is still eating fine then it might be worth taking some photos and taking them into an aquarium store (not a regular pet store as they won't know jack).