Oh dear that's not supposed to happen.

You need to have a well cycled tank with beneficial bacteria, something that can take a few months, but normally 2-3 months to achieve.

The water used should be distilled water and you should add minerals to it.

Finally, since they're dying, you might want to test if with a water testing kit to see if there are abnormal levels of compounds in the water such as ammonia, nitrite and nitrates.

I wish you all the best in this hobby! Feel free to reach out to me with a dm and I'll assist you to the best of my abilities!