Newly set up Aquarium, need some help with water quality

Hey community,

Just found this community earlier today and need some help. Last week i purchased this tank (starter kit that came with everything) for a good price and wanted to get back into aquariums. After filling up the tank with water and letting it "cycle", thought it would be a good time to add some fish.

I was handed some test stripes from a local shop and when tested, it seemed to pass all the tests. Last night i bought 8 fish (4 neon tetras, 2 Red-gold guppies and 2 algee eaters). Things seemed to be fine until i checked on them this morning.

All of the neon tetras had died. 2 of them were floating and two of them havent been counted for. As i prepare the decreased ones a proper burial at the upstairs bathroom, I wanted to reach out to the community and see what i did wrong, how to improve to sustain a healthy environment for them.

Please let me know any recommendations, clearly the information here isnt enough, so i can update the information when needed.

Thanks,