| I agree with Loman about the nitrates....and it would not really be a good thing in every aspect to have 0 nitrates, even if they are toxic. The reasoning for me saying that is because having nitrates present in a tank is the end result of having a cycled aquarium. The loss of your fish, coupled with the 0 nitrates IMHO is suspicious and I would look deeper into that.
When was your last water change? Was it just prior to doing the tests?
Have you treated your tank recently with an antibiotic or other medicine?
Are the fish dieing slowly or in groups?
What dechlor conditioner are you using in this tank?
What are the parameters of your tap water?
Is this tank planted? If so, are you using CO2? (sorry, cant remember your total situation anymore...old-timers disease I guess)
When the fish die, can you describe how you find them? Do they have color when they are dead? Do they seem 'slimey' at all? Do they appear to have died recently or do they appear like they have been dead for some time?
I am just starting to understand Kh a little bit better now, and I am not familiar with all of the test kits and test results since one brand will have a different result than another from what I understand. For example, my test kit, which is the Hagen/Nutrafin variety, asks me to keep adding drops of a solution until I can change the color of the water. I believe it is from blue to yellow or vice versa (again, old timers disease...I just tested it last night, which is sad that I cant remember). I then take that amount of drops, in my case it was 7 and multiply by 10 to get a Kh of 70. From my test booklet, this is a semi-low number and so that means I should have some ability to change the pH of my water with relative ease. This can be good and it can be bad...bad because if the Kh is too low, then the pH could be swinging wildly throughout the day, and night, which could lead to my fish being shocked by that and death could occur. I think it is possible that you could be having that problem and if that really is the case, you should be able to address that in a number of ways, other than using chemicals. Using CO2 in a tank can also change your pH, lowering it usually throughout the day. The one thing I have recently learned is that plants do not use CO2, or use it differently, at night time and so there needs to be some concern about how the pH will be affected and fluctuate.
Of course, I have to go back and preface all of this by saying they are all nothing more than educated guesses and so my suggestion would be to keep an open mind and not focus too intently on one thing or another. My methodology when I am having an issue in my own tanks is to seek out and identify a specific pattern which is occuring. In this way, I can normally figure out, or greatly narrow down, what the problem is and then try to fix it in the least invasive manner as possible...just in case I am wrong, of course. | 
Just Wondering 
