The guy in question is from that tank.

There are a couple mollies in there besides the guppies and a plecostomos. I do test the ammonia level and it always is around 0, but I've never checked Nitrite/Nitrate levels. I do weekly water changes of about 20% - 25%, if not more than once a week. The pH is also in the proper range and I added aquarium salt, stress coat, etc. when I first set up the aquarium and replace respectively when I do the water changes. The temperature is kept between about 78 and 80. Unfortunately, the fish bit the dust, or gravel, the day after I posted this. Anyways, I'm still worried because it looks like if it was bacterial, it has spread to another male. If not, then I've still got a problem with the mollies, perhaps, but they tend to associate with eachother for the most part.

I've purchased some anti-bacterial stuff and started using that. The water has gotten pretty cloudy, but I am not going to perform a water change for a couple more days for the sake of the medicine.

Please let me know what you think.

Thank you for your help,
hopefully he'll  get better within the week.

Maybe we should simply post a warning that the search function on this site is a waste of time. "Please go to Google, type in Guppylog and what one is looking for."

Let me add a tiny bit to miskairal's observation about fish waste and clean water. Ammonia is clear, toxic and we wouldn't see it in the water. I would guess nitrites and nitrates are also clear. The bacterial bloom, which may follow, can discolor the water.

Among the hardest things for aquarists (whatever their experience) to do is to save water for a week or two of seasoning and then to do 15-50% partial, weekly water changes. Heck, it is difficult finding household space where that water will not get too cold to use as changing water! That hopefully keeps the toxins, in a cycled aquarium, down and hopefully will keep the fish's immune systems robust and working.

Add too much garbage to the water and the fish will get stressed. If the fish gets stressed (just like with people) the immune systems get compromise. Then the fish (or people) get sick.

I just did a bit of a rant in a comment to the diary "fishies are fine" not because I'm angry with the diarist, who seems like a neat and thoughtful individual, but because I really am frustrated by how little information is given to new aquarists about setting up a tank. Many shops could do and offer more.

The shops could counter with, "How often do potential aquarists pick up one of the many beginner books and read even a chapter on setting up an aquarium." There is some truth to that, but they need to mention to the inquirer that such literature is there.

By the way, googling "setting up an aquarium" draws almost 2 million hits! Even most of the info  from articles on salt water aquariums would apply to freshwater tanks. 10 Easy Aquarium Setup Steps
http://saltaquarium.about.com/od/startinganaquarium/a/aa_aqsetupstep6.htm
is a good example.

It seems that the info is out there. Maybe we just need to point that out and then answer the follow up questions. Sort of like, I read the set-up questions on the Tax Preparation software and loaded it. It then took me parts of 4 days to fight through the questions, reloads, removing of conflicting software, glitches and near crashing of my computer. ;)

I'll shut up now and let you do a partial water change. :)