Well it seems that there is conflicting info regarding this, but I can tell you that I have several fish tanks with fish not dying or seeming to be stressed or anything at all, with nitrates above zero and below 40. So based on my experience, I am not worried about these levels of nitrate.
You should be worried, if you care about the fish, and I trust you do. I went into a conversation with Dr. Neale Monks about nitrate. There are few in this hobby who have the level of knowledge that Neale has, and I highly value him as an online friend who has taught me a great deal, and I have been keeping fish for over 30 years. The following is from our conversations concerning nitrate.
The effects of nitrate on aquarium fish has not been greatly studied, and most scientific studies relate to commercial food fish raised in outdoor ponds. But these studies have shown that nitrate does impact fish growth, and fry especially are susceptible. You might argue that this means mature aquarium fish are OK, but this would be false reasoning. If "x" affects fish at any stage, then "x" is clearly a serious health issue.
With regard to aquarium fish, there is evidence that nitrate does weaken the fish, so in time the fish are more susceptible to various issues like disease, which they should and normally would be able to fight off were it not for the weakening from the nitrate. Ammonia, nitrite and nitrate are all toxic to fish. The levels can vary, and the effect on the fish may vary (ammonia and nitrite are rapid acting) but all are toxic. Cichlid sources now list nitrate as a prime cause of diseases, and this group of fish has been found to be especially sensitive to nitrate.
Natural habitat waters of all of our fish have nitrates so low it would probably be impossible to measure them with our kits. In the fish, nitrate will act much like nitrite, making it more difficult for the blood to carry oxygen. Nitrate has been directly linked to issues like bloat (especially in cichlids, Malawi Bloat), hexamita (hole in the head), and shimmies (mollies especially). There are no actual visible signs of nitrate's detriment, but the effect over time weakens the fish leading to disease/death which is not directly linkable to nitrate, but was due to the long-term effects resulting from the high nitrate. And we are talking anything at or over 20 ppm.