If you have nitrites I doubt you have nitrates yet, sounds like you tank is halfway cycled and your mix of fish is not recommend, so that's 2 negative factors working against you.
You have the bb that converts ammonia -->nitrite but not the second bb that converts nitrite-->nitrate.
I'd do 25% every few days till you read n03, I commend you for seeking help, we will get this worked out and your tank will be healthy in no time.
Also great advice above! You are lucky that members are taking the time to be extremely helpful!
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