RE: Question about planted aquarium and fishless cycling
How are the rest of your fish?
Chain stores usually sell low quality fish. I have learned it the hard way too. My first batch of fish were of low quality, and most of them died to diseases and parasites (ick, gill fluke, mouth rot, etc.) showing up just after a few days after I got them. Some of them were just weak and drop dead for no visible illness.
When a problem occurs, there might not be any fix specialized for it. Sometimes the best thing you can do is to provide fish the excellent water quality and high quality food. Also remember do not overfeed. Most problems should be gone.
The highest quality fish I got were from Dr Frost and Smith's http://www.liveaquaria.com
They are more expensive, but their fish are so healthy and totally worth it.
For your plants, it needs more than just regular aquarium lights. I use T5HO lights on timer for 10 hours a day for my plants. You also need some nutrients for plants to do well. Special plant substrate and some aquarium plant tabs are used in my case. I also need to admit that having live plants mean a lot more trouble to run an aquarium, but totally worth it if you are good at "gardening".