Sick goldfidh-sarasa comet?

got this fish at the fair. kid named it perky turkey. we bought a 6-7 gallon tank with filter. noticed a vertical red line where tail meets body when we got it. fish was fine for 5 days, eating and swimming. day 6, sits at bottom of tank. day 7 not eating still at bottom. took water sample to fish store they said water is good, no problems. now what??