Looks a bit thin, is it on the other fish? And I know its a gold fish tank but do you have a heater? Stable temps are a good thing in any tank IMO even if its a cold water fish like them.
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i think i want a fish. maybe two.
Def.use the heater. Otherwise, the temp will not be stable, and that makes goldfish (especially fancy ones, like yours) prone to illness. I don't know about the white mark, but I would be concerned, if it were on my fish, too. Is there anyone out there with lots of goldfish experience, who has seen this before? (beautiful fish, btw)
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20 gal; well planted
2 angelfish
4 cherrybarbs
5 cardinal tetras
1 dwarf gourami
3 oto catfish
1 bristlenose pleco
1 crown tail betta
2 rasboras
1 cory cat
a few shrimp
White spot on wen
maybe two. 
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