HELP: One of my Leopard Danios is dying!

It could be any number of things. My first observation is that your
tank is not cycled. If it was cycled your ammonia would be 0, your
nitrite would be 0 and you would show some nitrates. When you have
alkaline water such as yours (pH of 8.0) the toxicity of ammonia
increases drastically. Even .25 ppm becomes dangerous. We need more
information to know what's actually causing his problem though.

What other fish do you keep with him? What kinds and how many?
What is the gh and kh of your water? Have you had any pH fluctuations?
What are the physical symptoms? Not eating? Lethargic? scrapes, nips or
parasites on his body?
How long have you had him?
What has he been eating?
What is the water temperature?
Have you had any mechanical problems? Heater, filter, air stone?
What water conditioners are you using? Are you adding anything else to
the water? Medication?
Anything else you can think of that may be helpful?