Ah ok, so the tank wasn’t cycled then. Cycling is essentially getting beneficial bacteria to start living in your tank and reducing the waste chemicals produced by aquatic life to a less harmful chemical (nitrogen cycle). Essentially the idea is you’d continuously introduce some sort of ammonia into the tank and monitor the water until it drops to 0.

This seems to be a good article to get started: https://www.freshwatersystems.com/blogs/blog/how-to-cycle-a-fish-tank

I’m sorry for the loss of your fish though. Aquariums can have a bit of a learning curve and we have all made mistakes. I hope this’ll be a sad but good learning experience and that you’ll continue to explore this hobby!

Letting the water "air out" often does not work anymore. Many municipalities have switched to chloramines for water treatment. Unlike chlorine, chloramines are long-term stable in water. This means they need to be removed either by treating the water with chemicals or filtered out with reverse-osmosis or activated carbon.

Since it is a new tank adding some beneficial bacteria like safestart would be good then letting it sit for at least two weeks without adding anything else. Take it for testing by a local aquarium shop and if it's all clear then add some fish. It sounds like the tank was not fully cycled unfortunately. Best of luck man don't let it get you too down, everyone has to learn somehow.