Common Goldfish from Festival?

Hi, I won a goldfish at the fair a week ago, I bought a bowl and food for it, and I boil water and let it cool to put in the bowl. The fish seemed rather lifeless and stayed at the bottom of the tank most of the time. He has black patches on his fins, he also had some on his scales, but that seems to have come off, taking the scales with it. I noticed he had a light patch on his tail and it looked like it was fin rot, It also looked like some of his fins were fraying around the edges. I bought 'Interpet fungus/fin rot 100ml' I put the designated amount in the tank and within a few hours my fish seemed more active and now swims round a lot more. The patch on his tale that went pale/clear-ish now has a grey/black tinge to the edge. What can I do ? I am continuing using the 'interpet' when i change the water. But what else can I do ?