Probably without meaning to, most of you are agreeing with me. "Stress" is indeed the cause of 95% of all fish deaths. In most cases, fish are able to overcome this and that, through their immune system when it is disease-oriented, or through their general "good health" when it is environmental (as in the temperature). The fact that the fish dies means it is under stress, so logically we look for possibilities.
I had missed the number factor, and there is no doubt at all that this causes stess. I am not blaming the member for not having more fish available, these things happen. But please do not deny the stress resulting. It is just there.
Nitrates above 20 ppm will affect most aquarium fish over time. Now, we will not get agreement on this, so there is no point in harping. But there is sufficient evidence from scientific studies to suggest that nitrates, like the counterparts ammonia and nitrite, do negatively impact fish physiology. The impact is not as drastic or sudden with nitrate as it is with ammonia and nitrite, but it is still there. And this may well be adding stress over time.
Unless we are trained ichthyologists, we cannot possibly know the internal state of our fish. All we can do is understand the issues and accept them. I have been battling a "disease" problem in one tank for over a year now; it has not so far killed a fish, but it is attacking their gills. It will suddenly appear, then after two weeks is dissipates. Obviously the treatment is retarding it but not "curing" it. I have a professional microbiologist helping me through this; one of the very first things she suggested for the recurrence after a couple months was stress, so we looked at every aspect of maintenance from water changes to foods to additives (for plants, water conditioner, etc) to parameters to species and numbers to...whatever--all with the aim of determining any likely stress. A little stress from "A" may not seem like much, but to the fish it may be an issue, and then "B" or "C" occurs to add more stress...next thing you know, fish are mysteriously dying. There really is no mystery in fish deaths, there is always a reason. It may be genetic or caused by something along the way. At some point an added stress which also might be of no consequence otherwise, is the straw that breaks the camel's back. Any time I see a member respond that this or that has existed for "x" months/years with no problem, I can only shake my head. There is a problem...you just can't recognize it. But to dismiss it out of hand is not wise.
Byron.