There are many possible reasons for fish flashing, and nitrate is one of them. But "high" nitrate can mean something very different to different members, so you/we need to know test results. Is there a fish store that will test? If yes, make sure they give you the number, not some vague and meaningless term like "acceptable." Nitrate in the source water needs to be investigated too, and here you might be able to find out from the water data; check the website of your water supplier, or call them.
I had a similar problem in one tank for over a year before I finally resolved it. With the help of two professional biologists, it turned out to be a very stubborn case of ich. No spots were ever seen on fish, but this is not surprising because ich first attacks fish in the gills where we cannot see it but it causes the flashing. I am certainly not saying this is the issue in your case, only pointing out that it cannot be ruled out just because you do not see spots. Velvet is similar. Or even some other parasitic protozoan.
Clean water is key in any of this; regular weekly water changes of at least half the tank but preferably 60-70% goes a long way for healthy fish. But stubborn cases of some parasite can still continue.