Obviously you have got an immature filter.
The first thing you need to do is to read
this article here. It will tell you about what needs to happen in your aquarium. Some of it is happening, but the rest of it isn't.
One thing you can do is to change water daily, to reduce the amount of nitrite in your tank. THis is hard work, and when I said daily, that's exactly what I meant
The other downside of this is that you are slowing down the process, because you are trying to grow bacteria in your filter, and water changes deprive those very bacteria of the food they need to grow.
WIth nitrite, you need to add salt to the tank to counteract the nitrite. The chloride atoms in salt enter the fish through the same part of the gill that the nitrite uses - with enough salt, the chloride will outcompete the nitrite for space at the entry point. You need to 10 times the amount of chloride, so you need about 15 times the amount of salt (obviously salt is partly sodium and partly chloride).
You haven't said what your nitrite figure is, so you need to multiply that figure by 15, and add that amount of milligrams of salt per litre of water in your tank.
Keep monitoring the nitrite level, if it increases by 50%, do a 50% water change, and add 50% of the amount of salt that you originally added.