My goldfish has some scales missing, what could it be?

I notice a few days ago, but my fish swims too fast and i thought it was the light. Btu today i saw it was like 3 more scales missing. It swims normal and eat well, maybe not as much as before, but it still eats well. I changed 50%of the water last Friday, added salt. This is it... show more I notice a few days ago, but my fish swims too fast and i thought it was the light. Btu today i saw it was like 3 more scales missing. It swims normal and eat well, maybe not as much as before, but it still eats well. I changed 50%of the water last Friday, added salt. This is it http://www.youtube.com/watch?v=iSst2euupmY It's been doing this since we put the tank in that place and somebody told me it could be ich or just it sees its reflex on the wall, which is true, you can see in the video. Since it swims like that JUST on that side of the tank but both ways and it's missing scales on one side of its body, i think it's not ich. What do you think? Should I add some melafix? It doesn't have any blood or fuzz.
The parameters of the tank are ok. It's just one goldfish and a mystery snail in a 10-gallon tank. Should I add some stress zyme or amonnia detoxifier anyways?
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