Sick Faintail Care

Thank you for you help in advance.

2 day before I brought 4 goldfish. 2 of them are fantail.

1 is Fantail already have black spots on it so may she is already sick.
She is just seating at bottom of tank and not eating anything.

I have test amonia and nitrate, PH and nitrite all test are good.

Only 1 fish from 4 is having problem all other are playing and eating well.

How should I care that sick fish as this is my first tank and I am worried about my fantail.

Please help me to make my fantail happy I just brough 2 days before and I don't want to return sick fantail because pet shop is not going to care much.