That suggests that there is still something in the tank leaching into the water column and causing the issue.Ok new test from tap water n yes I guess tap water comes with nitrites. Weird thing is that I fill my other tanks with the same water n the other tanks are fine n nobody is dying
Do you have a spare tub you can temporarily put all the fish in while you remove the substrate and start again with something like inert sand? I know it's a lot of work but might be worth trying so that you know there's nothing in the substrate leaching out.