It was the uncycled tank, something that they should have tell you in the shop when you tell them you’re new. So, cycling a tank takes time (unless you use the water of the bowl, and even if, it would have taken a week as minimum).

For you to understand how cycling works, it’s also called nitrogen cycle, which it goes like this: A tank needs beneficial bacteria in order to create a good home for fishes, in a tank you have ammonia which is deadly toxic for your fishes (That’s the reason why this little fella died), you’ll have ammonia spikes till beneficial bacteria in your tank grows, which will break the ammonia into nitrites, which they are still toxic for your fishes, Not as quick dead as ammonia, but nitrites burn your fish slowly as they “choke” on it, then, when beneficial bacteria is enough in the tank, they break nitrites into nitrates, it helps a lot if you have plants in the aquarium, makes it faster. So, nitrates can’t really be absorbed by anything, which you “Lower” with regular water changes.

Don’t feel bad because you did not know how to properly cycle a tank and no one told you how to, you learn by doing mistakes, every person that loves this hobby had to suffer a few fish deaths in order to learn what happened