IMHO, you must be having some sort of acclimation problem here as far as what I have read anyways. When you buy new fish, how do you add them to the tank? Most LFS will say that you should float the bag in the water to match the temperatures, but there is alto of other issues fish need to acclimate to as well and that is why I am a very firm believer in the ideas, tips, and tactics that are outlined in this article:
http://www.fishtankforums.com/6-salt...livestock.html
I realize that this is in the saltwater forum of this site, but honestly, any and all of the ideas can translate into freshwater and I am willing to bet that this would increase your odds of keeping new fish alive...even if that is not your true problem. My thinking here is that you have a pretty low pH, which is fine, but the pH at the store you purchase your fish from may be completely different. Freshwater fish have the ability to acclimate to a very wide range of pH levels, however, they cannot do it in an instant (such as going from the pH level of the water in the bag your bring them home in, right into your own tank).
I also did not see any mention of using a dechlor conditioner in your tank. I suspect that you do use one, but I just wanted to make sure....so do you? If so, what type?
For the 100% water changes...they are really not needed if you are doing weekly water changes. Eventually, you would replace all of the water over a succession of a month or so and so it is not needed to do a 100% water change ever. I agree with badkitty in thinking that it would stress out your fish, but I wouldnt think that the end result would be fish death unless you were really belligerent about how you go about doing so. Honestly though, there is no real benifit to doing that once a year and I would think it to be overly risky, at least IMO.
