I know that you expect us to know everything to tell you but truth is we already have. Your tank is cycling and fish die during cycling if there are too many like in your case. Google nitrogen cycle for fish tank and you should be off on a good start. Also it doesn't hurt to do your own research, google is an infinite source of info.
Also people will often ask for your water parameters so it would be worth while to invest in an API freshwater master test kit. Good luck!