When was the water tested? I'd be extremely surprised if all is now as it should be as the tank is very overstocked and it's likely ammonia or nitrite have risen as a consequence, hence the deaths. Unfortunately, it isn't big enough for even one goldfish .... see this article for reasons why:
https://www.fishkeeping.co.uk/articles ... -size-life-expectancy.htmThe best thing to do is return them to the shop and get something more suited to a small tank.....maybe White Cloud Mountain Minnows, though the tank being tall rather than long is not ideal for them either. If it was mine I'd add a heater and go for either a single male Betta splendens or a shoal of tiny tropicals such as mosquito rasbora or celestial pearl danios, depending on how hard your water is.