Nitrate is detrimental to aquarium fish at just about any level. Ammonia and nitrite poison fish more rapidly, but nitrate over time and depending upon the level and the species does cause problems. Generally it weakens the fish so it becomes more susceptible to other problems it should otherwise be able to handle without issue. Keeping nitrate as close to zero is always advisable. [I am not for a moment suggesting nitrate is the issue here, only pointing out that it is not helping.]
This is fairly easy to do provided nitrate is not already present in the source (tap) water. Regular (once weekly) substantial (60-75% of the tank volume) water changes, not overstocking or inappropriately stocking, not overfeeding, keeping the filter clean, substrate vacuuming, and live plants all help when nitrate is occurring solely from the biological system. And it should be easy to keep it in the low range, say 0 to 5 ppm.
When nitrate is present in the tap water, and at the level indicated here, it might be advisable to deal with that. Diluting with RO (or using just RO, depending upon the fish species) is one way. There are also filtration methods that other members have used with success.
@AbbeysDad is one who has battled quite high nitrate in his source water, he can advise on this aspect.
