I had the same problem a week ago. All the sudden way to high nitrites! My buddy gave me one of his filters and I replaced mine with it and a few days later the bacteria destroyed all the nitrites and almost all the nitrates. If you can manage to get someones established used filter, swap it. Then invest into getting another exact filter as the one your using and only replace one filter at a time. Replace one, then the other the nest time. Yes you'll have a really dirty filter but you'll also have the bacteria. For larger aquariums its a good idea. I'm doing now and I never even have to worry about nitrates. I do a water change once a month as well as a filter change. But swapping filters saved my fish man, and really helps to have friends using the same filters I do. Hope it helps. And don't use prime or any additives to get rid of it, they just mask it and it pops up worse when it wears off. They don't get rid of it like they say they do. 25% water change once a week. But not to much or you'll loose your bacteria and it will have to cycle again cause the same problem as before.
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