Whats killing my old fish when adding new ones?

Hello, i have a bi-orb (60l) no heater, nothing extra in it bar pebbles and an ornament i have 1 fancy goldfish. an oranda. The problem im having is whenever i go and get more fancy goldfish to add to the tank, my old ones die??!! this is very sad as i get very attached to them and i feel like ive killed them. i clean it on a regular basis or whenever i see algae on the tank wall, i use stress coat every time, as sad as it is i make sure each of them has had at least one sinking pellet in the morning and night, the fish i add are the same size and from pets at home (not diseased) i can clean my tank after they have settled but my old ones still die. i hate only having one fish in a 60liter tank but i just cant risk adding any more in. HELP