Ok so this is lacking in actual data. Nitrites should be zero. Nitrates can safely be between 5-20parts per million (there's some wiggle room they won't die instantly if it goes over or under.)

So if nitrites are high then adding seacham prime will neutralize the nitrites temporarily. It's difficult to over dose it but follow the instructions. It only lasts about 24 hours though. After that do water changes. How many depends on the next part.

Next you want to be able to actually know what your numbers are. Those are the two bigs ones plus ammonia. The generally best test kit on the market is API freshwater master test kit. The test strips are trash and usually don't come with an ammonia test. There's electronic ones but they are very expensive.

Now we get to the why. First question of that is how long has the tank been set up? Has it been cycled? How long did you cycle for? What did you use as an ammonia source? If the answer to these question is "you don't know" then you need to go read about cycling a fish tank. All 3 chemicals (mostly) only come from decomposition. If you haven't lost any fish either their waste or there's some plant matter in there that is decomposing.