I don't think nitrates in any of my FOWLR tanks have ever been under about 40 ppm. Other than some anecdotal evidence involving certain species, I'm not aware of any study that shows nitrates, at any level, bother fish at all. If nitrates did bother fish at a given level; I'm sure there would be studies to prove it; because of the money spent on nitrate reducing products. Bob Fenner, in "The Conscience Marine Aquarist' (1st ed.) talks about fish living with nitrates in the hundreds, even thousands of ppm; without problems. He then adds, of course, the lower the better. I have kept many tough fish for many years without even testing for nitrate ;prior to LR being used to control nitrate. I'm sure nitrate exceeded 100ppm at that time. However; I think excessively high nitrate can indicate an overall lack of good housekeeping . Grab a sample of your LFS and test it some time, you'll be amazed. BTW, LA suggests nitrate in a FOWLR tank not exceed 30 ppm and I see no reason to argue with that. Nitrates at say, under 10ppm, would be nice; but I'm not going to kill myself or my wallet getting there when it isn't necessary. All of the above drivel is for FOWLR tanks, of course.