try choose those goldfish tat doesnt seems like their weight imbalance. sumthing like the fish like losing is body balance n oways like to float. 2nd u can try to put u hand up to the aquarium. observe sum fish. healthy fish sure come up to eat. if those hide under water din come up better dun pick it.Others is tat try to c if ther is goldfish color. if its pale it may b sick. Den is c if any fin is broken or scale is lose. look for pop eye oso. oso c for the tail . if the tail got line red red like the urat i duno wat they called it.

Its like u buying fish at fish market. Takkan u buy an unhealty fish to eat.
sorry for my ingland