I'll assume that you are using city water supply and your are treating the water with some sort of conditioner... if it's Prime, then the nitrite will be rendered non-toxic for a day or two. I'd suggest not only doing the 50% change, but change some every day or two treating it again until the nitrite subsides... you don't mention ammonia, it will probably be high as well but Prime handles this... some conditioners don't do everything.