In many cases with elevated water parameters like nitrate, in combination with higher conductivity than is normal for a certain species, ill health is not immediately apparent, or acute.
Those off kilter parameters cause chromic, slowly creeping problems, like suscepability to certain bacteria (HLLE) or other even more subtle maladies, giving the impression of mystery disease or worse.
I know many people think I'm blowing this stuff out of my ***, and that I blather on and on about the toxicity of "less toxic" nitrate, but its effects are cumulative (especially when combined with other not so natural water, for a species) but it is because its effects are so subtle, why it is often overlooked.