Goldfish are easy (mostly because they're pretty tough and survive a broader variety of conditions than a lot of other fish) but still harder than most people think XD

_{are they young goldfish one of them looks like a guppy}

Did you do the put the bag in the water first when you put them in the tank or did you plop them in? Do you have an aerator in the tank? I got told by one person that the smaller tanks don't need them but then someone else told me that all tanks regardless of size and presence of plants need them so I've been putting them even in bowls (not that I had bowls after reading that they were not ideal either x_x) just because the bubbles looked cool.

Another possibility is Cheetah was already coming down with something at the shop and when you brought him home the stress from the environment change made it worse, or maybe it would have gotten worse anyway, and hopefully it'll be fine in a few days.

Good luck! :S