I'm disappointed too, because so few new aquarists will take the trouble to try and "fishlessly" cycle an aquarium, adding a set amount of ammonia periodically and testing every day or so, until the ammonia readings are 0, the nitrite readings are close to 0 and the nitrate readings may be increasing but are under 20 PPM. Do you know what those readings were before the fish were added? What are those readings now?

It is refreshing to run across someone doing the fishless cycling. Too many new people think that if they let the water sit for a week or two, that the tank is cycled and biologically active. That sitting is not a bad thing in itself because a fair amount of CO2 and nitrogen will be released and the water will absorb a little oxygen. Indeed as the weather gets colder in the Northern Hemisphere, new water (as it warms from the tap to room temperature) is actually sometimes super-saturated with gases and fish can die of gas embolisms (not too different from the bends.) But if letting water sit is all that is done for the   fish, it still has to cycle over the next six weeks or so.

Starting with just a couple of guppies is wise. We often start with too many.

Just out of curiosity, did you get the idea from Angelhologram's article in Immediate Help?

Another thing which takes out new fish is abruptly dumping them in the tank. Given your conscientiousness with the cycling, I wouldn't think that would have been what happened to your male guppy. For those looking on, for more on acclimating the new fish and not inoculating the tank (heavily anyway) with diseases from the fish shop, please see "Getting Acquainted: Acclimating New Fish to a New Aquarium" in the New Tank/Cycling/Setting Up/Water Changing of Immediate Help.

I'm assuming that you have been feeding your guppies no more than they could clean up in a couple of minutes, If they showed no sign of disease, this is a bit of a mystery. Even with diseases, the fish really needs to be to a laboratory pathologist within minutes of death, so it is hard to guess the cause of death.

Wish I could be more help.

If you wish to speculate further, the very same person who wrote that fishless cycling article also offered up a checklist of things to consider if something goes wrong. (Some old geezer added a few questions). You might wish to peruse 20 (40) Questions for new fish owners or owners of sick fish which will be found at http://www.guppylog.com/story/2005/6/24/82111/0134

Those are things which a lot of us will ask of ourselves and others if something hasn't worked out in an aquarium.

Administrivia:
By the way, like so many of us as first time posters, you opted to submit a log, which according to the site software and FAQs is a major exploration of some aspect of the hobby. These Opuses will run from several hundred to several thousands of words on Guppylog. On other Scoop sites, logs may be longer!

That is a design issue of the software. We're not trying to put on airs.

I think you will agree that questions like yours would make great diaries. :) And, because they aren't stuck in the queue and don't have to get voted upon, they get posted immediately to the diary and everything lists and very likely, get responded to sooner.