I'm not sure what you should do, maybe you should bleach it? Did it have a filter? Did you disinfect that?
Oh, I noticed you said you got a goldfish :( In the long run, he would have died anyways because all goldfish get massive, and need at least 20 gallons to get to adult size. In such a tiny tank, he would have stopped growing, but his intestines would have continued growing until they were too big, then he'd die. I don't want to sound morbid, but I thought you might want to know why not to put goldfish (no matter how small) in tanks less than 20 gallons.