I agree your nitrates were way too high, and that certainly needs addressing.
On another note, just to be thorough, the only times I have had experiences where everything just died without looking sick, it was temp/heater related.
How's your heater doing? and don't just answer "oh it's brand new so it's awesome" like everyone does. Go check the temp over the course of the day and night. I've had them stick on and cook everything, or look on and not be and everything died, or plain old crack and electocute the **** out of everything (this one was very exciting when I reached into the tank to get the dead fish out - I yelled some bad words).
What I am trying to get across is always be wary that there may be more than one issue. Always try look at everything and you will have fewer surprises.
Finally, losing everything in your tank frickin blows donkey arse. Sorry.