3 weeks? You tank is still cycling. It'll take some time for the bacteria that converts nitrite to nitrate to set in.
Secondly, you have fishes in there already. They are probably contributing to the high nitrite. If its a planted tank, you can let the tank cycle without fishes. Then slowly add them in to let the tank ecology adjust to new sources of ammonia
From your numbers, it looks like your filter is fine, cos ammonia is being converted to nitrite. So give a few more weeks.
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