I have a 55 gallon freshwater tank that we purchased in December.  For the last couple of weeks, we have had several fish die (a couple a day) for no apparant reason.  We have checked the water religiously.  We did have a battle with ick about a month ago, but we seem to have taken care of it and my husband is a fanatic about doing the water changouts.  Not too often, but just right.  We can tell which ones aren't doing well because they tend to slow down and stay in one spot until we find them dead.  There was one yesterday that my husband and I looked at and he almost looked like he had a hunchback all of a sudden.  The water is testing fine.  We have plents of snails and algae eaters.  One thing that I wonder about is the fact that we have two bubblers.  One has a section that is somewhat sealed and that is where the water passes through.  It is black with mold and I am wondering if it is putting something dangerous in the water.  Again, I know my water is good, we have the right amout of fish for our tank and we keep our water at the temperature it is supposed to be at.  None of these things are the issue.  We have added some new fish recently to replace some that we lost.  We always put them in a quarantine tank for a while before adding them to the big tank, but it is the established fish that seem to be dying.  If anyone has any ideas, please let me know as I am at a loss.  I don't want to continue to replace fish (they are expensive) and what's more, I like the fish I have and I don't want to lose them.  Please share some advice with me and help me save my fish!!!

It sure sounds like you know what you are doing.

Some questions:

1. What does the PH test out at?
2. What make and model of filter do you use?
3. What are the species of fish that are dying?
4. Do the fish die before, or after or  between water changes?