I didn't say that it was anyone else's fault...not sure what you're talking about. I was pointing out that you said you overfed and the logical issues that come after overfeeding is excess food collecting in the substrate of the tank and rotting. I am fully aware that fungus happens naturally in a tank, but you dont SEE it unless it has something to grow on (ie extra food, dead fish bodies, decaying plant matter, sick fish).
I read through all of their posts and all of your posts...Lotus pointed out that one of the best ways to tell about your tank is through testing, as well as asked if your tank is cycled...which you didn't answer. Seleya too pointed out that having toxins in the water is a bad thing and suggested that testing your water might be a good idea. I realize you read their posts and may have responded to pieces of them, but you didn't seem to note those parts that a lot of us find really important to the basics of keeping fish.
I dont think you need to disinfect anything, but thats your call. There are many people that go through fish deaths and diseases without ripping the tank apart because they realize that poor water quality was their issue. If you do decide to disinfect make sure to use a very dilute bleach solution (like 100 to 1) then rinse well and use a lot of dechlorinator to deactivate any excess bleach.
Ammonia and nitrites are not visible to your eye...ick and fungus and fish deaths are most commonly attributed at least in part to poor water quality. Having ammonia and nitrites are two things that would make your water "poor quality" and since you had an overstocked tank that has only been set up for less than a month (
http://www.myfishtank.net/forum/showpost.php?p=279967&postcount=21 ) it is reasonable to assume that it is still cycling and that is why I'm telling you that this DID come from poor water quality.
I hadn't responded before this on this thread because it seems like no matter how hard I try to explain some of this stuff you've been ignoring it...and I have been very frustrated reading your posts. I realize that you're new to the hobby and maybe sometimes I dont explain things that well...but when there is a good handful of people telling you the same thing and you dont seem to notice I'm not sure what else we can do.