help fish start dying

[Madmak]

IMO, and just from watching the video, something must be way off. Please get a test kit for the basics and maybe copper too. Test the tank so you know where you are at and continue with water changes but test the new salt water as well. Could be anything causing the problem and you will find it if you keep looking. Bad salt, bad fresh water, low oxygen, high nutrients, heavy metals, etc.

Best of luck to you.