healthy fish

I'm new to a 'larger' fishtank. I've been keeping a betta for several years. I had salt water tanks when I was a kid (my parents did but I did a lot of the water changing and feeding and such) and I considered getting a salt water tank but the fish are expensive and require a lot more time and...

I'm an experienced fresh water keeper. I am looking for a reason my entire tank of fish have died within the past two days. The only change to the tank that I made was to add live plants. Is that a possibility that the plants killed the fish? The fish previously we're all very healthy, well...