new to forum dont think I got the right area but need to ask some questions about tank nitrites. I have a 75 gal tank established for about 23 years now with one pleco fish. Been lucky no real issues over the years. This week did my cleaning testing all were in range expect the nitites level reading above .3mg/l no2 nutrafin test kit. did the 20% water change added water cond. double dose of cycle and used salt. next day reading the same did another water change the next day reading are still the same. I dont know what to do am iam worried for my fish. Any thoughts would be great.