HELP: One of my Leopard Danios is dying!

Hmm, well that's curious. Your tank seems to be in order.

It is possible that the tank was going through a mini-cycled if you
added another fish but I don't think that has anything to do with your
danio.

My best guess is, since you've only had him a week, that whatever was
wrong with him may have started before you purchased him. As you likely
know, the majority of fish die in the first two weeks in their new
home. It's a very stressful time and he may have had any number of
diseases made worse by the stress of a new tank. (Here I should
emphasize the importance of quarantining your fish for 4 weeks prior to
introducing him to your community tank.)

I'm sorry to hear you lost the little guy. I wish I could give you a
more definitive answer, but it sounds like you're doing everything
you're supposed to be doing.