Usually fish flicking and rubbing themselves against objects in the tank indicates one of these problems;
a. Fish could have flukes, if they are body flukes then they will not be visible to the naked eye, however gill flukes often make fishes gills appear sore/red or swollen looking. Flukes are an external parasite that often cause just flicking/rubbing symptoms in fish as far as symptoms go of the parasites.
b. Fish could have the beginnings of whitespot, although the classic symptoms of whitespot are small sand grain size whitespots on the fish, the parasite goes through numerous life cycles, not all of which cause visible external symptoms, only just behavioral ones like flicking/rubbing and labored breathing etc.
c. There are irritants in the tanks water like undesirable water quality conditions or unwanted chemicals that have accidentally got into the tank like soap/cleaning chemicals, room sprays, deodorants etc.
It is kinda weird that you have no nitrates in the tank, most established tanks will have some nitrates in them since nitrates are the end product of the nitrogen cycle, and it is uncommon to have absolutely no nitrates unless the tank is very well planted or is finishing off its cycle. How long has this tank been set up for and what is your tank cleaning regime like/how do you go about it?
Also, have you introduced any new fish to your tank recently and what are the fish and numbers of fish you currently have in your tank
? How many gallons/liters is the tank as well?
I'm betting my money on that the fish have flukes, a type of external parasite, a lot of fish in fact carry flukes however face no obvious issues with them, its when the fish becomes stressed or its health goes downhill a bit for whatever reasons that the flukes go out of control and the fish can no longer deal with them, the fish then tries to itch the flukes off by rubbing itself against objects however this can sometimes just cause of the fish even more stress or even make it injure itself etc. If one fish is infested with flukes then they can spread to other healthy fish in the tank so its very important to treat the fish for flukes if this turns out to be what the fish has- more info on your situation would be handy in helping find out what exactly your fish have
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